feat(model-manager): add "Delete all" gallery button + clearer 401 errors

- Gallery: DELETE /api/gallery/all removes every image under output/;
  "Delete all" button with in-app confirm and a deleted/failed count.
- Downloads: surface a clear, actionable message when CivitAI/HuggingFace
  returns 401/403 (model requires login/early-access, or the key/token
  lacks access) instead of a bare error, both at resolve time and during
  the download stream.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

model-manager/app/downloader.py

@@ -111,8 +111,11 @@ async def run_download(download_id: int, url: str, headers: dict[str, str],
         db.update_download(download_id, status="canceled", error="Canceled by user")
         _cleanup(part_path)
     except httpx.HTTPStatusError as exc:
-        db.update_download(download_id, status="failed",
-                           error=f"HTTP {exc.response.status_code}")
+        code = exc.response.status_code
+        msg = f"HTTP {code}"
+        if code in (401, 403):
+            msg += " — requires login/early-access or invalid API key"
+        db.update_download(download_id, status="failed", error=msg)
         _cleanup(part_path)
     except Exception as exc:  # noqa: BLE001 - surface any failure to the UI
         db.update_download(download_id, status="failed", error=str(exc))