add interactive chats to the Home canvas

[morgmart]

Category: new-feature
User Impact: Users can expand pinned chats into live, interactive conversation cards on Home without leaving the canvas.

Problem: Opening a pinned chat currently replaces the Home canvas, breaking spatial context and making it harder to keep several active conversations in view. Solution: This opt-in experiment lets pinned chats expand in place with a bounded recent-history window and the shared composer, while full chat remains available for complete history and deeper work.

Refs #143

Product behavior

• Manually enable Chat on canvas in Settings → Experiments; it remains off by default and is excluded from development auto-enable.
• Click a pinned chat to expand it in place. Expansion, size, and position persist.
• Each card shows the latest 10 complete user-led exchanges; older history has an explicit Open full chat boundary.
• Read, scroll, select text, send, queue, steer, stop, and attach files without changing the route-selected chat.
• Every expanded card keeps its composer visible. Focus remains ephemeral and only deliberate interaction marks a session read.
• Drag expanded cards from the header only; transcript and composer gestures remain owned by chat.
• Collapse the card or maximize into the complete chat workspace from the header.

Architecture

• Extracts shared chat-owned transcript and composer capabilities used by both full chat and Home.
• Keeps dispatch, queueing, admission, security confirmation, read-only, and separate-window rules in the existing session controller.
• Uses a Home-only bounded projection and classic renderer for reliable compact cards; full chat keeps complete virtualized history.
• Adds reference-counted transcript retention so mounted cards are not evicted from the session cache.
• Persists expanded/collapsed presentation through the existing Home layout schema and experiment registry.

Verification

• just check
• Full Vitest suite: 6,880 passed, 1 skipped (last full run before final metadata/icon-only changes)
• Focused experiment tests: 49 passed
• Focused canvas/header tests: 81 passed
• Manual dogfood: multiple expanded chats, recent-history boundary, send/queue/stop, selection and scrolling, header-only drag, resize, pan/zoom, collapse/open full chat, and reload persistence

File changes

src/app/AppShell.tsx / src/app/ui/AppShellContent.tsx
Keep session dispatch route-neutral and pass the shared composer/session callbacks into Home.

src/features/chat/capabilities/ConversationComposerCapability.tsx
Extract the shared conversation composer authority and presentation policy used by full chat, Home, and canvas cards.

src/features/chat/hooks/useChat.ts
Keep session-addressed sends and compaction from changing route selection.

src/features/chat/hooks/useChatTranscriptReadModel.ts
Provide a read-only, session-addressed transcript model for non-route chat surfaces.

src/features/chat/hooks/useSessionAddressedComposerAdmission.ts
Centralize session failure, read-only, security, and separate-window admission across chat surfaces.

src/features/chat/lib/boundedConversationProjection.ts
Project the ten most recent complete user-led exchanges for the in-the-moment canvas window.

src/features/chat/stores/chatStore.ts
Retain independently mounted transcripts safely through cache trimming and session-ID promotion.

src/features/chat/ui/ChatInput.tsx
Support the shared composer’s bare embedded presentation.

src/features/chat/ui/ChatTranscriptSurface.tsx
Extract the reusable transcript/loading/artifact surface from the route workspace.

src/features/chat/ui/ChatView.tsx
Compose the full workspace from the shared transcript and composer owners.

src/features/chat/ui/MessageTimeline.tsx / VirtualMessageTimelineGate.tsx
Support bounded start content and owning-surface renderer selection.

src/features/experiments/experimentDefinitions.ts
Register Chat on Canvas as manual-only and off by default.

src/features/home/lib/homeLayoutMapper.ts
Persist expanded/collapsed chat presentation and per-profile size memory.

src/features/home/ui/HomeComposer.tsx
Adopt the shared composer capability without changing Home’s pending-chat behavior.

src/features/home/ui/HomeView.tsx
Hydrate and compose pinned chat sessions for Home.

src/features/home/ui/WidgetCanvas.tsx / WidgetFrame.tsx
Own ephemeral canvas-chat focus, contained wheel gestures, lifecycle, and header-only drag behavior.

src/features/home/ui/useWidgetDragSuppression.ts
Suppress activation after cancelled pointer gestures.

src/features/home/widgets/ChatCanvasCard.tsx
Add the interactive expanded card: project identity, bounded transcript, composer, history boundary, and card controls.

src/features/home/widgets/ChatPinWidget.tsx
Expand chat pins in place under the experiment while preserving stable default behavior.

src/features/home/widgets/catalog.ts / types.ts
Define expanded chat sizing profiles and widget integration contracts.

src/shared/i18n/locales/en/*.json / es/*.json
Add experiment, card action, history-boundary, loading, and unavailable copy.

Changed test files
Cover composer parity and admission, bounded exchange projection, transcript retention, route neutrality, renderer ownership, experiment registration, layout persistence, multi-card lifecycle, drag/scroll/focus behavior, and bounded-history performance.