feat: add OSC 52 clipboard support for container-to-host copy

Programs inside the container (e.g. Claude Code's "hit c to copy") can
now write to the host system clipboard. A shell script shim installed as
xclip/xsel/pbcopy emits OSC 52 escape sequences, which the xterm.js
frontend intercepts and forwards to navigator.clipboard.writeText().

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/src/components/terminal/osc52.test.ts

@@ -0,0 +1,59 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+
+/**
+ * Tests the OSC 52 clipboard parsing logic used in TerminalView.
+ * Extracted here to validate the decode/write path independently.
+ */
+
+// Mirrors the handler registered in TerminalView.tsx
+function handleOsc52(data: string): string | null {
+  const idx = data.indexOf(";");
+  if (idx === -1) return null;
+  const payload = data.substring(idx + 1);
+  if (payload === "?") return null;
+  try {
+    return atob(payload);
+  } catch {
+    return null;
+  }
+}
+
+describe("OSC 52 clipboard handler", () => {
+  it("decodes a valid clipboard write sequence", () => {
+    // "c;BASE64" where BASE64 encodes "https://example.com"
+    const encoded = btoa("https://example.com");
+    const result = handleOsc52(`c;${encoded}`);
+    expect(result).toBe("https://example.com");
+  });
+
+  it("decodes multi-line content", () => {
+    const text = "line1\nline2\nline3";
+    const encoded = btoa(text);
+    const result = handleOsc52(`c;${encoded}`);
+    expect(result).toBe(text);
+  });
+
+  it("handles primary selection target (p)", () => {
+    const encoded = btoa("selected text");
+    const result = handleOsc52(`p;${encoded}`);
+    expect(result).toBe("selected text");
+  });
+
+  it("returns null for clipboard read request (?)", () => {
+    expect(handleOsc52("c;?")).toBe(null);
+  });
+
+  it("returns null for missing semicolon", () => {
+    expect(handleOsc52("invalid")).toBe(null);
+  });
+
+  it("returns null for invalid base64", () => {
+    expect(handleOsc52("c;!!!not-base64!!!")).toBe(null);
+  });
+
+  it("handles empty payload after selection target", () => {
+    // btoa("") = ""
+    const result = handleOsc52("c;");
+    expect(result).toBe("");
+  });
+});