Infinite Canvas — Phase 7 (deferred): Native webview panels (Browser + App preview)

[Legacynnn]

Part of the Infinite Canvas epic: #61

Phase 7 (deferred) — Native webview panels: Browser + App preview

Add in-canvas browser and running-app preview panels. Deferred because of a hard macOS platform constraint.

⚠️ Hard constraint

Native webviews (WKWebView) paint above the host DOM on macOS. So browser/app-preview panels:

• cannot be occluded by other (DOM) panels,
• cannot honor the global translucency slider,
• ignore canvas z-order and tldraw chrome.

This phase positions native webviews over their canvas rect and documents/handles these limitations rather than hiding them. (Reference the existing browser-pane approach + occlusion notes in the repo, e.g. TOOL_PILL_BAND_PX / content-host handling.)

Scope

• Browser panel — native webview keyed per panel (per-pane registry, label like canvas-browser-{panel_id}), URL bar, basic nav/history. A connected conversation can navigate / screenshot it.
• App-preview panel — native webview pointed at the running dev app; connectable to a conversation.
• Move/resize repositions the native webview rect; entering/leaving canvas mode and switching workspaces correctly shows/hides/tears down webviews.
• Document the occlusion/translucency limitations in-app (subtle hint) and in the epic.

Optional follow-up

Spike screenshot-streaming / off-screen rendering so these panels become fully composable DOM panels (heavier/slower) — separate issue if pursued.

Acceptance

• Browser + app-preview panels render, navigate, move/resize, and tear down cleanly across mode/workspace switches.
• A connected conversation can drive/screenshot them.
• Limitations documented; lint/typecheck/test pass.