Arrange the window the way VS Code arranges one

[rajanbor]

Change

The shell was a sidebar and one full-width surface. Reading an agent while looking at the sandbox it runs in meant clicking between them and holding the first one in your head — and everything in this app is an object worth putting next to another object.

Three columns now, the arrangement VS Code uses:

Column	What it is
Activity strip	One icon per area — agents, search, source control, sandboxes, models, workflows — with settings and the account at its foot. The lit icon marks the window's edge; clicking it again collapses the sidebar.
Sidebar	Whichever area is lit, in collapsible sections, resizable by its right edge.
Work area	Tabs in groups. Any tab splits to the right, drags into another group, closes with the middle button or ⌘W. An emptied group folds away unless it is the last, which shows a watermark.

web/src/lib/layout.ts owns the model: a tab is a key, a view, a title, an icon and the id of the object it points at; every operation returns a new layout and none mutates its argument. Tabs come from named constructors (agentTab, sandboxTab, modelTab…), so a title and an icon are decided once wherever the view is opened from. Opening a key that is already on screen focuses it rather than repeating it. Three groups is the limit — beyond that no pane is readable, so the split control disappears instead.

The arrangement is written to this machine's storage and read after mount, so the first render still matches the prerendered HTML. A restored tab pointing at an agent or sandbox the engine no longer has is dropped rather than rendered.

Two sidebar areas are new rather than moved:

• Search reads the objects this window already holds — agents, their chats, sandboxes, models, workflows — and says plainly that file contents need the daemon.
• Source control shows branch, working tree and recent commits without going through a top-bar menu.

Shortcuts: ⌘B sidebar · ⌘\ split · ⌘W close · ⌘1–⌘6 areas, alongside the existing ⌘K, ⌘J, ⌘I.

Validation

• pnpm build (style check, Next build, TypeScript) clean.
• In the browser: opened an agent, split it right and confirmed two groups render side by side with their own strips; reloaded and the two groups came back; clicked through agents, models and usage; checked light and dark.
• No new console errors. (The console carries older errors from a dev-server state that predates this branch; the count did not move during these interactions.)

Security impact

None. Presentation and local layout state only — no credential, command or network path. The stored layout holds object ids and titles, nothing more, and is read defensively: unreadable storage falls back to the default layout in silence.

Closes #60.