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.

backend/src/backend/scripts.py

@@ -1114,6 +1114,29 @@ async def list_scripts(
     }
 
 
+# 工作区内 active 脚本总数。DashboardRoute 等不需要列表但需要计数的场景使用,
+# 避免被 listScripts 的懒加载语义污染。该路由必须在 /scripts/{script_id} 之前声明
+# ——FastAPI 按声明顺序匹配,否则 `count` 会被当作 script_id 命中 get_script。
+@router.get("/api/v1/scripts/count")
+async def count_scripts(
+    context: RequestContext = Depends(request_context),
+    session: AsyncSession = Depends(database_session),
+) -> dict[str, Any]:
+    total = await session.scalar(
+        select(func.count())
+        .select_from(Scripts)
+        .where(
+            Scripts.workspace_id == context.workspace.workspace_id,
+            Scripts.status == "active",
+        )
+    )
+    return {
+        "request_id": context.request_id,
+        "data": {"total": int(total or 0)},
+        "meta": {},
+    }
+
+
 # 读取脚本正文或 Notebook JSON；编辑器打开文件时调用此接口。
 @router.get("/api/v1/scripts/{script_id}/content")
 async def get_script_content(