feat(scripts): GET /api/v1/scripts/count + DashboardRoute wiring

After #34 the workspace store only holds the root-level scripts plus
whatever subfolders the user has expanded. DashboardRoute's
"全部脚本"/"工作副本" counts derived from scripts.length therefore
underreport the workspace total until the user navigates to /scripts
and expands every folder.

Fix: separate count endpoint + dedicated store field, mounted
independently.

Backend — backend/src/backend/scripts.py
- New endpoint GET /api/v1/scripts/count.
- Route declared BEFORE /api/v1/scripts/{script_id}/... so FastAPI's
  declaration-order matching does not interpret "count" as a script_id.
- Returns { data: { total: number }, meta: {} }; SQL is a single
  COUNT(*) on scripts filtered by workspace_id + status='active'.

Frontend — services/api.ts + context/AuthContext.tsx
- countScripts(workspaceId) client; WorkspaceBoundApi gains the field;
  AuthContext binding forwards workspaceId.

Frontend — state/scriptWorkspaceStore.ts
- scriptCount: number | null, scriptCountLoading: boolean.
- loadScriptCount() action: idempotent (no-op while in-flight), silent
  on failure (dashboard tolerates a stale count).
- Initial state and reset() clear both fields.

Frontend — features/platform/DashboardRoute.tsx
- Subscribes to scriptCount; calls loadScriptCount() on mount.
- Falls back to scripts.length until the count resolves so the
  dashboard never blanks.

Tests — backend/tests/test_count_scripts.py (new)
- 3 unit tests: scalar result handling, NULL coercion, route callable.

Verified: pytest 55 passed (52 + 3 new); pnpm typecheck clean.

frontend/app/context/AuthContext.tsx

@@ -222,6 +222,7 @@ export function useApi(): WorkspaceBoundApi {
 
   return useMemo<WorkspaceBoundApi>(() => ({
    listScripts: (parentPath) => rawApi.listScripts(workspaceId, parentPath),
+   countScripts: () => rawApi.countScripts(workspaceId),
    listResources: (opts) => rawApi.listResources(workspaceId, opts),
    createScript: (input) => rawApi.createScript(workspaceId, input),
     uploadScript: (file, parentPath, visibility) =>