Add browser-style tab bar for working across multiple T3 Code sessions

[ShpetimA]

Scope of changes:

• Users can switch from the sidebar to Top tabs in Settings. The sidebar remains the default, and compact/mobile layouts continue using it.
• Threads, new-message drafts, Settings, Usage, and Pull Requests can stay open in separate tabs, so users can move between active work without losing context.
• A dedicated new-tab page makes it easy to start work from recent projects and threads.
• Tabs can be reordered, closed, and navigated with familiar shortcuts such as Cmd/Ctrl + T, Cmd/Ctrl + W, and Cmd/Ctrl + 1–9.
• Tabs communicate important state at a glance, including active work, unsent drafts, and snoozed threads. Users can also quickly wake snoozed threads from the tab bar.
• Thread actions are available directly from tabs: pin, rename, snooze, settle, archive, or close a thread. Closing a tab is designed not to accidentally discard ongoing work.
• When using full-width tools such as Diff, Terminal, Browser, or Files, the chat remains available as a tab alongside those tools, making it easier to switch between discussion and implementation context.

T3tab-lofi.mp4