Shell polish: storage line, status pill, footer strip, sidebar

Backend:
- apps.core with GET /api/status/ (env, git hash, OS, watched folder,
  worker counts) and a TimingMiddleware adding X-Server-Time-Ms
- status endpoint reports its own server-side assembly time

Frontend:
- top bar shows the storage line (watched folder) and a status pill
  with ENV, git hash, host OS, server time (e621 time once it exists)
- fixed 32px footer strip: storage path, active workers, build version
- collapsible filter sidebar: closed by default, Ctrl/Cmd+B toggle,
  overlay drawer with backdrop blur under 1024px
- rating filters, per-page and sidebar state persist in localStorage

backend/apps/core/views.py

@@ -0,0 +1,44 @@
+import platform
+import time
+
+from django.conf import settings
+from rest_framework.permissions import AllowAny
+from rest_framework.response import Response
+from rest_framework.views import APIView
+
+
+def _os_info():
+    system = platform.system()
+    icon = {"Linux": "linux", "Darwin": "apple", "Windows": "windows"}.get(
+        system, "unknown"
+    )
+    return {"name": system, "release": platform.release(), "icon": icon}
+
+
+def _worker_counts():
+    """Active/queued background jobs.
+
+    No task pipeline exists yet, so this reports honest zeros for now.
+    """
+    return {"active": 0, "queued": 0}
+
+
+class StatusView(APIView):
+    """Runtime status used by the SPA shell (storage line, status pill, footer)."""
+
+    permission_classes = [AllowAny]
+
+    def get(self, request):
+        started = getattr(request, "start_time", time.perf_counter())
+        payload = {
+            "app": "J621",
+            "env": settings.APP_ENV,
+            "git_hash": settings.GIT_COMMIT_HASH,
+            "version": settings.APP_VERSION,
+            "os": _os_info(),
+            "watched_folder": settings.WATCHED_FOLDER,
+            "e621_time_ms": None,  # no e621 calls in the backend yet
+            "workers": _worker_counts(),
+        }
+        payload["server_time_ms"] = round((time.perf_counter() - started) * 1000, 1)
+        return Response(payload)