Canvas / A2UI

Goal

Canvas can show a bundled A2UI shell at /__clawdis__/a2ui/ when no session index.html exists.
The A2UI shell forwards a2ui.action button clicks to native via WKScriptMessageHandler (clawdisCanvasA2UIAction).
Native forwards the click to the gateway as an agent invocation.

Close button: render a small vibrancy/material pill behind the “x” and reduce the button size for less visual weight.
Click reliability:
- Allow A2UI clicks from any local Canvas path (not just / or the built-in A2UI shell).
- Inject an A2UI → native bridge at document start that listens for a2uiaction and forwards it:
  - Prefer WKScriptMessageHandler when available.
  - Otherwise fall back to an unattended clawdis://agent?...&key=... deep link (no prompt).
- Intercept clawdis://… navigations inside the Canvas WKWebView and route them through DeepLinkHandler (no NSWorkspace bounce).
- GatewayConnection auto-starts the local gateway (and retries briefly) when a request fails in .local mode, so Canvas actions don’t silently fail if the gateway isn’t running yet.
- Fix a crash that made clawdis-mac canvas show/eval look “hung”:
  - VoicePushToTalkHotkey’s NSEvent monitor could call @MainActor code off-main, triggering executor checks / EXC_BAD_ACCESS on macOS 26.2.
  - Now it hops back to the main actor before mutating state.

Add a small “action sent / failed” debug overlay in the A2UI shell (dev-only) to make failures obvious.
Decide whether non-local Canvas content should ever be allowed to emit A2UI actions (current stance: no, for safety).