fix: route sign-in links by what can actually catch the callback

`isAnthropicSignInUrl` made the container the default action for every
Anthropic sign-in link, justified by "the host has nothing to catch it
with". That was wrong in both directions. The host does have something --
the auth bridge -- and the container side is not a general browser at all
but Playwright's dashboard, whose packages and chromium are deliberately
not baked into the image. So the default pointed at the one path that is
uninstalled on a fresh project, on every platform, while the path that
works sat behind a switch.

The decision now lives in `useSignInOpenTarget`: a live auth bridge picks
the host, otherwise a container that can actually launch a browser picks
the container, otherwise the host. It resolves at mount rather than when a
URL arrives, so the buttons do not swap under a moving mouse, and it
re-decides on `auth-bridge-changed` so flipping the switch during a
hanging login takes effect. A bridge with port conflicts reads as not
live; an empty `active_ports` does not, since there is nothing to bridge
until the CLI binds its listener and that races the URL.

Both buttons still render either way -- this changes which one leads.
`sanitizeRelayUrl` is byte-for-byte unchanged, so the embedded copy in
web_terminal/terminal.html needs no matching edit.

The host "Open" path also failed silently: `dismissUrlPrompt()` ran before
`openUrl`, so the toast vanished and a rejected promise reached only the
devtools console. Dismissal now happens on success only, leaving "In
container" one click away after a failure, and the error surfaces through
the same toast the container path already used. On Linux this catch will
not fire for the common case -- `xdg-open` routinely exits 0 having done
nothing -- so it complements the AppImage environment fix rather than
replacing it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

app/src/lib/browserViewSupport.ts

@@ -0,0 +1,53 @@
+/**
+ * What a container has to have before anything can be opened *inside* it.
+ *
+ * The Browser tab asks this to decide what to offer; the terminal's URL toast
+ * asks it to decide which of its two buttons should lead. Both need the same
+ * answer, so the predicates live here rather than beside either caller — the
+ * failure this avoids is the toast steering a user at a container-side browser
+ * that the Browser tab is, on the very same screen, offering to install.
+ *
+ * The important thing to know about `PlaywrightDetection` is that browsers are
+ * deliberately **not** baked into the image: the libraries they link against
+ * are, the binaries are a user-pressed install. So "Playwright is present" and
+ * "a page can actually be opened" are two different questions, and a fresh
+ * project answers yes to neither.
+ */
+
+import type { PlaywrightDetection } from "./types";
+
+/**
+ * Mirrors Rust `PlaywrightDetection::is_usable` — the packages the live
+ * dashboard needs. Says nothing about whether a browser exists to show in it.
+ */
+export function isBrowserViewUsable(d: PlaywrightDetection | null): boolean {
+  return d !== null && d.playwright_version !== null && d.has_bind && d.cli_entry !== null;
+}
+
+/**
+ * Whether `openPageInContainerBrowser` has a browser to launch.
+ *
+ * Stricter than {@link isBrowserViewUsable} on purpose: the packages can be
+ * installed with `~/.cache/ms-playwright` still empty, which is exactly the
+ * state a `playwright install` step exists to leave behind, and launching into
+ * it fails several seconds after the click.
+ *
+ * Unknown reads as "no". A probe that could not run (stopped container, an
+ * image predating these fields) leaves the executable fields absent, and the
+ * caller's fallback — the host browser — is the one that at least reports its
+ * own failure. Over-refusing costs a user one extra click on a button that is
+ * still right there; over-accepting costs them a sign-in that goes nowhere.
+ */
+export function canOpenPageInContainerBrowser(d: PlaywrightDetection | null): boolean {
+  if (!isBrowserViewUsable(d) || !d) return false;
+  // The viewer's own Chromium, confirmed on disk by the probe.
+  if (d.chromium_executable_exists) return true;
+  // Google Chrome is an apt package, so it is never in `browsers` and has no
+  // revision to skew against.
+  if (d.chrome_channel !== null) return true;
+  // `== null`, not `=== null`: a probe from a container predating the
+  // executable fields omits them entirely, and `undefined` there means "didn't
+  // answer", not "missing". In that case a non-empty bundle list is the only
+  // evidence available, and it is better than nothing.
+  return d.chromium_executable == null && d.browsers.length > 0;
+}

app/src/lib/urlRelay.test.ts

@@ -4,6 +4,7 @@ import {
   MAX_RELAY_URL_LENGTH,
   RelayRateLimiter,
   URL_RELAY_OSC,
+  isAnthropicSignInUrl,
   parseUrlRelayOsc,
   sanitizeRelayUrl,
   urlOrigin,
@@ -321,3 +322,53 @@ describe("RelayRateLimiter", () => {
     expect(rl.allow("https://c.example/", 10_200)).toBe(true);
   });
 });
+
+describe("isAnthropicSignInUrl", () => {
+  // Classification only. Where a sign-in link should be opened is decided by
+  // `hooks/useSignInOpenTarget.ts`, from facts about the project — this answers
+  // the narrower question of whether it is a sign-in link at all, and it does
+  // so through the same allowlist the sign-in flow itself uses.
+  it("recognises the links `claude setup-token` and `claude login` print", () => {
+    expect(
+      isAnthropicSignInUrl(
+        "https://claude.ai/oauth/authorize?code=true&client_id=abc",
+      ),
+    ).toBe(true);
+    expect(
+      isAnthropicSignInUrl("https://platform.claude.com/oauth/code/callback?x=1"),
+    ).toBe(true);
+    expect(isAnthropicSignInUrl("https://console.anthropic.com/login?x=1")).toBe(
+      true,
+    );
+  });
+
+  it("is not fooled by a host that merely contains an allowed domain", () => {
+    // The thing the allowlist exists for: `claude.ai.evil.tld` ends with
+    // neither `claude.ai` nor `.claude.ai`.
+    expect(isAnthropicSignInUrl("https://claude.ai.evil.tld/oauth/authorize")).toBe(
+      false,
+    );
+    expect(isAnthropicSignInUrl("https://notclaude.ai/login")).toBe(false);
+  });
+
+  it("holds the full validator, not just the host test", () => {
+    // It runs `sanitizeRelayUrl`, so everything that cannot be opened at all
+    // is not a sign-in link either — no separate, weaker copy of the rules.
+    expect(isAnthropicSignInUrl("javascript:claude.ai/login")).toBe(false);
+    expect(isAnthropicSignInUrl("https://claude.ai@evil.tld/login")).toBe(false);
+    expect(isAnthropicSignInUrl("https://claude\nai/login")).toBe(false);
+  });
+
+  it("does not claim every allowlisted URL is a sign-in", () => {
+    expect(isAnthropicSignInUrl("https://claude.ai/chat/abc")).toBe(false);
+    expect(isAnthropicSignInUrl("https://www.anthropic.com/news")).toBe(false);
+  });
+
+  it("leaves an ordinary link alone, whatever it says in its path", () => {
+    // A `gh auth login` device code is the common one, and sending it to a
+    // container-side browser would be actively wrong.
+    expect(isAnthropicSignInUrl("https://github.com/login/device?code=A")).toBe(
+      false,
+    );
+  });
+});

app/src/lib/urlRelay.ts

@@ -182,11 +182,22 @@ export function extendsUrl(next: string, current: string): boolean {
 /**
  * Whether this is a URL that signs the user in to Anthropic.
  *
- * Used to decide *presentation*, not permission — the toast makes the
- * container-side browser the default action for these, because the OAuth
- * callback listener is inside the container and the host has nothing to catch
- * it with. It is deliberately the same host allowlist the sign-in flow itself
- * uses, so the two cannot disagree about what a sign-in link is.
+ * Classification only. It answers "is this a sign-in link", never "where should
+ * it be opened" — that decision moved out to `hooks/useSignInOpenTarget.ts`,
+ * because it depends on things this module has no business knowing: whether the
+ * project's auth bridge is live, and whether a browser is actually installed in
+ * the container. This function stays here because the *rule* it encodes is a
+ * URL rule, and it is deliberately the same host allowlist the sign-in flow
+ * itself uses, so the two cannot disagree about what a sign-in link is.
+ *
+ * It used to carry the default with it — container-side always, on the grounds
+ * that "the OAuth callback listener is inside the container and the host has
+ * nothing to catch it with". Both halves of that are now wrong. The host does
+ * have something to catch it with (the auth bridge mirrors the container's
+ * loopback listener onto the same host port), and the container-side target is
+ * not a general browser but Playwright's dashboard pane, whose browsers are
+ * deliberately not baked into the image — so on a fresh project the default
+ * pointed at something that was not installed, on every platform.
  */
 export function isAnthropicSignInUrl(url: string): boolean {
   const safe = sanitizeRelayUrl(url, { allowHosts: ANTHROPIC_SIGN_IN_HOSTS });