Notification click switches to the tab; pulsing question indicator on tabs

[SergKam]

Two ergonomics improvements.

1. Notification click → switch to the originating tab

Previously, clicking a desktop notification only raised the window. Now it also switches the notebook to the workspace tab the notification is about.

• A new application action activate-workspace (string target = workspace id) is registered in build_ui. Its handler resolves the id to its tab index (under a short borrow, released before set_current_page so the switch-page handler can borrow mutably), switches to it, and calls window.present().
• notify_workspace sets this as the notification's default action with the workspace id as target, and now keys notifications by workspace id so a newer alert for the same tab replaces the older one instead of stacking.
• The installed com.flycrys.app.desktop matches APP_ID, so action routing works for the running instance.

2. Tab shows a question indicator while the agent waits

While the agent is blocked on an AskUserQuestion, the tab's spinner is swapped for a pulsing amber question-mark icon, so a tab that needs you is distinguishable at a glance from one that's still working. The spinner returns the moment the agent resumes — on answer, reject, or a new message — and on finish.

• A gtk::Image question icon is threaded through the same path as the existing tab spinner: TabSlot → Workspace::new → create_agent_panel → PanelState.
• The pulse is a CSS @keyframes animation (.tab-question) in both light and dark themes. If a GTK build ignores @keyframes, it degrades gracefully to a static amber icon — still a clear signal.

Notes / caveats

• Not visually verified — these are UI-visual changes; clippy (-D warnings, lib+bins) and the full test suite pass, but the spinner↔question swap and the notification click should be eyeballed in a running session.
• Notification action delivery depends on a running session bus; under the multi-instance NON_UNIQUE flag, routing targets the live instance (the normal single-instance case).