feat: show copyable session ID in session detail header (#28)

## Summary 

- Adds a clickable session ID to the session detail breadcrumb bar
- Shows first 8 characters of the session ID in a monospace styled
element
- Hovering shows the full session ID as a tooltip
- Clicking copies the full session ID to clipboard with "Copied!"
feedback (1.5s)
- Extracted a reusable `copyToClipboard` utility with tests

This lets users easily grab session IDs for `claude --resume <id>`.

## Demo


https://github.com/user-attachments/assets/b7ef65c8-d80a-40c7-8a90-13d5e049c494

## Tests Ran

- [x] `copyToClipboard` utility tested (success + failure paths)
- [x] All 383 frontend tests passing
- [x] Go tests passing (no backend changes)
- [x] Manual: navigate to session, verify truncated ID visible
- [x] Manual: hover shows full ID tooltip
- [x] Manual: click copies full ID, shows "Copied!" for 1.5s

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Wes McKinney <wesmckinn+git@gmail.com>

Author: 3 people

frontend/src/App.svelte

@@ -16,8 +16,16 @@
   import { ui } from "./lib/stores/ui.svelte.js";
   import { router } from "./lib/stores/router.svelte.js";
   import { registerShortcuts } from "./lib/utils/keyboard.js";
+  import { copyToClipboard } from "./lib/utils/clipboard.js";
   import type { DisplayItem } from "./lib/utils/display-items.js";
 
+  let copiedSessionId = $state("");
+
+  function sessionDisplayId(id: string): string {
+    const idx = id.indexOf(":");
+    return idx >= 0 ? id.slice(idx + 1) : id;
+  }
+
   let messageListRef:
     | {
         scrollToOrdinal: (o: number) => void;
@@ -191,6 +199,25 @@
                   })}
                 </span>
               {/if}
+              {#if session.agent === "claude" || session.agent === "codex"}
+                {@const rawId = sessionDisplayId(session.id)}
+                <button
+                  class="session-id"
+                  title={rawId}
+                  onclick={async () => {
+                    const ok = await copyToClipboard(rawId);
+                    if (ok) {
+                      const id = session.id;
+                      copiedSessionId = id;
+                      setTimeout(() => {
+                        if (copiedSessionId === id) copiedSessionId = "";
+                      }, 1500);
+                    }
+                  }}
+                >
+                  {copiedSessionId === session.id ? "Copied!" : rawId.slice(0, 8)}
+                </button>
+              {/if}
             </span>
           {/if}
         </div>
@@ -295,4 +322,22 @@
     white-space: nowrap;
     flex-shrink: 0;
   }
+
+  .session-id {
+    font-size: 10px;
+    font-family: "SF Mono", "Menlo", "Consolas", monospace;
+    color: var(--text-muted);
+    cursor: pointer;
+    padding: 1px 5px;
+    border-radius: 4px;
+    background: var(--bg-tertiary);
+    transition: color 0.15s, background 0.15s;
+    white-space: nowrap;
+    flex-shrink: 0;
+  }
+
+  .session-id:hover {
+    color: var(--text-secondary);
+    background: var(--bg-hover);
+  }
 </style>

frontend/src/lib/utils/clipboard.test.ts

@@ -0,0 +1,21 @@
+import { describe, it, expect, vi } from "vitest";
+import { copyToClipboard } from "./clipboard.js";
+
+describe("copyToClipboard", () => {
+  it("copies text and returns true on success", async () => {
+    const writeText = vi.fn().mockResolvedValue(undefined);
+    vi.stubGlobal("navigator", { clipboard: { writeText } });
+    const result = await copyToClipboard("test-id");
+    expect(writeText).toHaveBeenCalledWith("test-id");
+    expect(result).toBe(true);
+    vi.unstubAllGlobals();
+  });
+
+  it("returns false when clipboard write fails", async () => {
+    const writeText = vi.fn().mockRejectedValue(new Error("denied"));
+    vi.stubGlobal("navigator", { clipboard: { writeText } });
+    const result = await copyToClipboard("test-id");
+    expect(result).toBe(false);
+    vi.unstubAllGlobals();
+  });
+});

frontend/src/lib/utils/clipboard.ts

@@ -0,0 +1,8 @@
+export async function copyToClipboard(text: string): Promise<boolean> {
+  try {
+    await navigator.clipboard.writeText(text);
+    return true;
+  } catch {
+    return false;
+  }
+}