Task: Fix AI design history loading stall

Identity

Replace full-history AI Design conversation loading with a cursor-paginated latest-message flow.
Ensure workspace history becomes visible before generation-task reconciliation completes.
Make rapid conversation switching safe when an event-stream connection is still pending.
Update the local and Works Square adapters, shared boundary, main-process route, renderer store/canvas, and focused tests as needed.

Initial conversation reads return the newest 10 message records; scrolling upward requests older pages and preserves viewport position.
Do not retain or transmit an unbounded message history through the normal conversation API, mutation payloads, or snapshots.
Preserve the existing workspace/conversation identity checks and do not apply stale responses after a switch.
A pending connection attempt is treated as owned until it settles or is explicitly aborted; returning A -> B -> A must not create duplicate pending streams for A.
Server and desktop client contracts must remain explicit and bounded; page cursor contents are opaque to the renderer.

The renderer shows a selected Conversation as soon as its Workspace response arrives; delayed or failed generation-task reconciliation can no longer keep the history view on its loading state.
Conversation history is cursor-paginated end to end: the initial page is the newest 10 messages, scrolling upward loads one older page at a time, de-duplicates stable message IDs, and retains viewport position.
Local and Works Square adapters expose the same page metadata. The Main route forwards the opaque cursor without interpreting it.
Rapid A → B → A switching reuses an in-flight A event-stream attempt instead of opening another non-cancellable request. Renderer disconnection while Main is still opening a relay now closes the eventual subscription, and an upstream WebSocket open is bounded by a 10-second timeout.

pnpm test — 183 files and 2167 tests passed.
pnpm exec vitest run tests/unit/image-workspace-api.test.ts tests/unit/image-workspace-store.test.ts tests/unit/local-image-workspace.test.ts tests/unit/works-square-design-workspace.test.ts tests/unit/image-workspace-route.test.ts tests/unit/image-canvas-page.test.tsx --reporter=dot — 154 passed. The Canvas suite still reports three pre-existing React act(...) warnings in its stale-auth test.
pnpm run typecheck — passed.
pnpm exec eslint electron/api/routes/image-workspace.ts electron/image-workspace/local-workspace.ts electron/image-workspace/module.ts electron/image-workspace/works-square-workspace.ts shared/image-workspace.ts src/lib/image-workspace.ts src/pages/ImageCanvas/index.tsx src/stores/image-workspace.ts tests/unit/image-workspace-route.test.ts tests/unit/image-workspace-store.test.ts tests/unit/local-image-workspace.test.ts tests/unit/works-square-design-workspace.test.ts tests/unit/image-workspace-api.test.ts — passed.
git diff --check — passed.

Main-branch promotion is pending release of the repository-wide integration lock held by task 20260819-merge-performance-package-6e5a.