feat: add per-tab workspace authentication infrastructure (#8073)

## Summary
Add workspace authentication composables and types for per-tab workspace
isolation. This infrastructure enables users to work in different
workspaces in different browser tabs.

## Changes
- **useWorkspaceAuth composable** - workspace token management
- Exchange Firebase token for workspace-scoped JWT via `POST
/api/auth/token`
  - Auto-refresh tokens 5 minutes before expiry
  - Per-tab sessionStorage caching
- **useWorkspaceSwitch composable** - workspace switching with unsaved
changes confirmation
- **WorkspaceWithRole/WorkspaceTokenResponse types** - aligned with
backend API
- **firebaseAuthStore.getAuthHeader()** - prioritizes workspace tokens
over Firebase tokens
- **useSessionCookie** - uses Firebase token directly (getIdToken())
since getAuthHeader() now returns workspace token

## Backend Dependency
- `POST /api/auth/token` - exchange Firebase token for workspace token
- `GET /api/workspaces` - list user's workspaces

## Related
- https://github.com/Comfy-Org/ComfyUI_frontend/pull/6295

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8073-feat-add-per-tab-workspace-authentication-infrastructure-2e96d73d3650816c8cf9dae9c330aebb)
by [Unito](https://www.unito.io)

---------

Co-authored-by: anthropic/claude <noreply@anthropic.com>
Co-authored-by: Amp <amp@ampcode.com>
Co-authored-by: Simula_r <18093452+simula-r@users.noreply.github.com>

src/locales/en/main.json

@@ -2592,5 +2592,20 @@
       "completed": "Completed",
       "failed": "Failed"
     }
+  },
+  "workspace": {
+    "unsavedChanges": {
+      "title": "Unsaved Changes",
+      "message": "You have unsaved changes. Do you want to discard them and switch workspaces?"
+    }
+  },
+  "workspaceAuth": {
+    "errors": {
+      "notAuthenticated": "You must be logged in to access workspaces",
+      "invalidFirebaseToken": "Authentication failed. Please try logging in again.",
+      "accessDenied": "You do not have access to this workspace",
+      "workspaceNotFound": "Workspace not found",
+      "tokenExchangeFailed": "Failed to authenticate with workspace: {error}"
+    }
   }
 }