fix(web): stamp focused-app context on main-composer messages

[mgoldsborough]

Problem

A user viewing an app (e.g. collateral) and typing into the main chat panel got an agent that couldn't see what they were looking at — it would say things like "I can't read your UI state directly — collateral doesn't expose a 'currently viewing' pointer."

The bundle is innocent: collateral does publish its current view via useVisibleState (activeDocument {id,name} + a summary). The break is in the web shell.

Root cause

The globally-mounted chat panel ChatChrome sent messages with no appContext:

// before
(text, files) => chat.sendMessage(text, undefined, files)

The only path that stamped app context was AppWithChat.handleChat — the in-app "ask about this" channel — which fires only when the iframe itself initiates a send, not when you type into the side panel.

The backend confirms the consequence: runtime.ts gates its entire focused-app resolution on if (request.appContext) and does not derive the focused app from X-Workspace-Id. So no appContext → no focused app → no "user is currently viewing X" section and no injected app visible state. Exactly the symptom.

Fix

A small standalone FocusedAppContext:

• The active AppWithChat publishes its AppContext while mounted and clears it on unmount / route change.
• ChatChrome reads it and stamps it on outgoing messages. useChat already enriches appContext with the app's latest visible state from the bridge store, so the open-document pointer now reaches the agent.

Standalone context (not folded into ShellContext) so focus changes re-render only the two participants, not every shell consumer. Non-app routes publish null → no appContext, unchanged behavior. The in-app channel and its [App Context: …] text prefix are untouched.

Test

web/test/FocusedAppContext.test.tsx pins the publish→read contract across two components (the exact link that was broken), the inert out-of-provider default, and clear-on-null.

Verification

• bun run verify:static — pass
• bun run test:web — 462 pass / 0 fail
• cd web && bun run build — clean

(The unrelated test:unit dompurify failure is a missing nested dependency in src/bundles/automations/ui/, present on a fresh checkout regardless of this change.)