feat(web): add embeddable thread route /embed/thread/:environmentId/:threadId

[rororowyourboat]

Summary

Adds a standalone route that renders the existing `ChatView` without the app sidebar chrome, for use inside iframes. The primary consumer is t3-canvas (a canvas-first agent workbench) which embeds T3 Code threads as tiles on an infinite canvas.

Closes #1

Changes

• New route: `apps/web/src/routes/embed.thread.$environmentId.$threadId.tsx`
  • URL: `/embed/thread/:environmentId/:threadId?minimal=1`
  • Renders `` with the existing bootstrap checks (environment connected, thread exists)
  • Shows "Loading thread…" while the environment bootstraps
  • Shows a clear error state for invalid thread refs or missing threads
  • Wraps in a `position: fixed; inset: 0` container with `data-t3-embed` attributes for future targeted CSS
• Root route update: `apps/web/src/routes/__root.tsx`
  • Any pathname starting with `/embed/` now renders `` directly instead of wrapping in ``
  • The environment connection, websocket surface, toasts, and event router still initialize — only the sidebar/diff/plan chrome is skipped
  • Auth gate still enforced (unauthenticated users see the outlet per existing behavior)
• `routeTree.gen.ts` regenerated by `@tanstack/router-plugin` (via `bun run dev` tick)

Follow-ups not in this PR

The `minimal=1` query param is parsed and exposed via `data-t3-embed-minimal="true"` on the container, but the actual chrome hiding (BranchToolbar, PlanSidebar, ThreadTerminalDrawer) inside `ChatView` is not yet implemented. These components render by default. Two reasonable paths, pick later:

1. Add a `minimal?: boolean` prop to `ChatView` that conditionally skips those sections
2. Scoped CSS on `[data-t3-embed-minimal="true"]` hiding `[data-branch-toolbar]` etc. (requires adding stable data attributes to those components)

For the MVP of t3-canvas agent tiles, showing the full ChatView inside an iframe is acceptable — the iframe is a large tile so the chrome is visible but not blocking.

Test plan

• `bun fmt` ✅ passes
• `bun lint` ✅ passes (0 warnings, 0 errors)
• `bun typecheck` ✅ passes (all 8 packages, ~37s)
• Manual: `bun run dev`, create a thread in the main UI, navigate to `http://localhost:/embed/thread//?minimal=1`, verify the thread renders without the sidebar and messages stream normally
• Manual: embed the URL inside a plain `<iframe>` on another local-origin page, verify same-origin iframe interaction still works (clicks, form input, scrolling)
• Manual: hit `/embed/thread/bogus/bogus` → verify graceful error state

Out of scope

• Chrome hiding for minimal mode (see follow-ups above)
• Cross-origin iframe hardening (same-origin only for MVP)
• Auth handling customization (unauthenticated users get whatever the root route serves)
• Multi-thread views or embed customization beyond `minimal`

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Handle unknown embed environments instead of perpetual loading

This gate returns the loading state whenever bootstrapComplete is false, but unknown environmentId values never bootstrap (the store falls back to an initial environment state with bootstrapComplete: false). As a result, /embed/thread/<bogus>/<bogus> never reaches the not-found branch and spins forever, which breaks the route’s intended graceful error behavior for invalid thread references.

Useful? React with 👍 / 👎.

[chatgpt-codex-connector]

Scope thread error writes to the thread environment

setError updates only state.activeEnvironmentId, but ChatView calls it with just a threadId for the currently viewed route thread. In multi-environment usage, when the viewed thread belongs to a non-active environment, this write targets the wrong environment slice (or no-ops), so transport/send errors are not persisted on the actual thread the user is looking at.

Useful? React with 👍 / 👎.