Slice 2 wave 3: wire chat end-to-end at the composition root

- app/store.svelte.ts: one WebSocket carries surfaces AND chat (onChat ->
  chatStore.handleDelta); build the conversation cache over the IndexedDB
  adapter; createChatStore wired to transport (socket.send), injected HTTP
  historySync, and the cache; load() on construct
- app/resolve-http-url.ts: host-relative HTTP base (port 24203), mirrors
  resolve-ws-url; injected fetch
- App.svelte: render ChatView + Composer alongside the surface picker
- createAppStore gains optional injection points (httpUrl/fetchImpl/indexedDB/
  conversationId) for tests
- vitest-setup.ts: fake-indexeddb/auto for jsdom IndexedDB (orchestrator-owned
  config; agent change adopted)

Verified green (x2, stable): svelte-check 0/0, vitest 218, biome clean, build ok.
Slice 2 (conversation transcript: cache + delta streaming) feature-complete.

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