feat(search-backend-selection): Search backend dropdown in Settings

- New features/search-settings: pure view-model (options = available backends
  ∪ 'None (search disabled)', unconfigured entries disabled with a suffix,
  unknown persisted selection prepended, network-seam normalization) + the
  SearchBackendField dropdown (save-on-change, spinner/saved/error states,
  revert-to-persisted on failure, 503 -> disabled field with a 'search not
  available' hint, loading skeleton, help text noting search-mode-only scope).
- store.svelte.ts: searchBackendConfig() / setSearchBackend() over the global
  GET/PUT /search/config routes (503 marked unavailable on the failure variant).
- SettingsPane renders the field under the chat limit; ExtensionsPane lists
  the new module manifest.
- .dispatch mirrors: wire + transport-contract deltas for the new types/routes.

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