👷 ci: Type-check the Client Workspace (#13560)

The `client/` workspace was never type-checked: the existing typecheck
job only covered `packages/` and `api/`, and Vite/esbuild transpiles
without type-checking, so type errors shipped through every CI gate.

- Add a `typecheck` job to frontend-review.yml running `tsc --noEmit`
  over `client/` (zero tolerance), reusing the data-provider +
  client-package build artifacts. Triggers on `client/**`,
  `packages/client/**`, `packages/data-provider/**`.
- Fix all 168 pre-existing client type errors this surfaced (source +
  tests), including genuine latent bugs:
  - `getFileConfig()` was typed as merged `FileConfig`, but the server
    returns the raw config that `mergeFileConfig()` consumes (`TFileConfig`).
  - SidePanel/Agents `Retrieval`/`ImageVision` were bound to `AgentForm`
    but use the assistants `Capabilities` enum → `AssistantForm`.
  - `useSearchResultsByTurn` read a `sources` field its type lacked.
  - Removed orphaned dead code: `Artifacts/Mermaid.tsx` (imported a
    never-installed dep) and dead barrel re-exports (`./Plugins`, `./MCPAuth`).
- Narrow `client/tsconfig.json` to the client app (drop `../e2e` and
  `../config/translations`, which reference backend/tooling modules) so
  the gate's scope matches its trigger.

No `any`/`@ts-ignore`/`as unknown as`. Localized newly-surfaced strings.

.github/workflows/frontend-review.yml

@@ -4,6 +4,7 @@ on:
   pull_request:
     paths:
       - 'client/**'
+      - 'packages/client/**'
       - 'packages/data-provider/**'
 
 permissions:
@@ -76,6 +77,50 @@ jobs:
           path: packages/client/dist
           retention-days: 2
 
+  typecheck:
+    name: TypeScript type checks (client)
+    needs: build
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Use Node.js 24.16.0
+        uses: actions/setup-node@v4
+        with:
+          node-version: '24.16.0'
+
+      - name: Restore node_modules cache
+        id: cache-node-modules
+        uses: actions/cache@v4
+        with:
+          path: |
+            node_modules
+            client/node_modules
+            packages/client/node_modules
+            packages/data-provider/node_modules
+          key: node-modules-frontend-${{ runner.os }}-24.16.0-${{ hashFiles('package-lock.json') }}
+
+      - name: Install dependencies
+        if: steps.cache-node-modules.outputs.cache-hit != 'true'
+        run: npm ci
+
+      - name: Download data-provider build
+        uses: actions/download-artifact@v4
+        with:
+          name: build-data-provider
+          path: packages/data-provider/dist
+
+      - name: Download client-package build
+        uses: actions/download-artifact@v4
+        with:
+          name: build-client-package
+          path: packages/client/dist
+
+      - name: Type check client
+        run: npm run typecheck
+        working-directory: client
+
   test-ubuntu:
     name: 'Tests: Ubuntu (shard ${{ matrix.shard }}/4)'
     needs: build