feat(web): add embeddable thread route /embed/thread/:id for canvas tile hosts #1

Goal

Add a minimal standalone route that renders just the conversation + input for a given `threadId`, with no sidebar chrome. This is the iframe target for t3-canvas agent shapes.

Context

t3-canvas is a canvas-first workbench that embeds T3 Code agent sessions as tiles. Each tile is a tldraw shape containing an iframe. For the iframe host, we need a URL that renders only the conversation view — no app-level header, no sidebar, no thread list. Query param `?minimal=1` toggles the stripped-down layout.

This patch is a prerequisite for t3-canvas issue #3 (Phase 2A).

Acceptance criteria

• New route `/embed/thread/:id` reads `threadId` from URL params
• Renders only the conversation component (messages, streaming, tool calls, input box)
• Query param `?minimal=1` hides any remaining chrome (padding, borders, app header)
• Works with existing WebSocket / provider session plumbing
• Missing thread shows a clear error state with a "recreate" action
• Existing routes and app-level chrome untouched

Scope

~200 LOC.

Files (verify against current repo state)

• `apps/web/src/routes/EmbedThread.tsx` (new)
• `apps/web/src/App.tsx` — register route
• `apps/web/src/components/EmbedThreadView.tsx` (new)
• CSS for `?minimal=1` mode

Branch

`feat/embed-thread-route` (separate from current `feat/sidebar-project-groups`).

How to test

1. `bun run dev`
2. Create a thread via main UI, note its ID
3. Visit `http://localhost:/embed/thread/?minimal=1`
4. Verify stripped-down view, streaming, input all work
5. Embed in an iframe from another local-origin page and verify the same behavior