Reorder tabs by dragging, and pop the browser view into its own window

Two things the UI couldn't do: rearrange the tab strip, and watch the
browser while working somewhere else.

**Drag to reorder.** `moveTab`/`moveActiveTab` on the store, HTML5 drag on
the strip with a marker showing where the drop lands, `Ctrl+Shift+←/→` for
the same thing without a mouse. Reordering deliberately does not select
what it moves, so a drag aimed at a background tab doesn't yank the main
area away from a terminal mid-run. A tab being renamed is not draggable —
a draggable ancestor swallows the mouse-drag that selects text in its
input.

**Pop the browser view out.** `browser_view/popout.rs` opens the view's
existing token-bearing loopback URL as a second OS window, with a
"Keep on top" toggle so it can float above the app. Window-only: the
viewer, the proxy and the container are untouched, so popping out and
back interrupts nothing.

Three things it rests on:

- No capability lists that window, so it has no IPC surface — right for a
  page served out of a container, and it must stay that way.
- The app CSP is irrelevant to it: `frame-src` constrains what the app's
  document may *embed*, and this is a top-level document. The port range
  and the token gate are what actually protect it, unchanged.
- The window is owned by the session, so the supervisor's teardown closes
  it. A window onto a viewer that no longer exists is worse than none.

The pane drops its iframe while popped out — two viewers can both *drive*
the browser, and two cursors on one page is not a feature.

`lib.rs`'s `on_window_event` is now guarded on `label() == "main"`. It
fires for every window and its body stops every container and exits, so
without the guard closing a pop-out would quit the app.

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

app/src/hooks/useKeyboardShortcuts.ts

@@ -11,6 +11,7 @@ import { useTerminal } from "./useTerminal";
  *   Ctrl+Shift+W  close the active tab
  *   Ctrl+Tab      next tab (Ctrl+Shift+Tab for previous)
  *   Ctrl+1..9     jump to the nth tab
+ *   Ctrl+Shift+←/→  move the active tab along the strip
  */
 export function useKeyboardShortcuts() {
   const { open: openTerminal, close: closeTerminal } = useTerminal();
@@ -51,6 +52,17 @@ export function useKeyboardShortcuts() {
         return;
       }
 
+      // Ctrl+Shift+←/→ — move the active tab, the keyboard route to what
+      // dragging a tab does. Shift is what keeps it clear of the terminal:
+      // Ctrl+←/→ is readline's word-wise cursor motion.
+      if (e.shiftKey && (e.key === "ArrowLeft" || e.key === "ArrowRight")) {
+        if (!state.activeTabKey) return;
+        e.preventDefault();
+        e.stopPropagation();
+        state.moveActiveTab(e.key === "ArrowLeft" ? -1 : 1);
+        return;
+      }
+
       if (e.shiftKey) return;
 
       // Ctrl+1..9 — jump to tab