Fix macOS sidecar build and blank window on startup

macOS sidecar: `uv run` re-resolves dependencies using CUDA sources
even after `uv sync --no-sources`. Use UV_NO_SOURCES=1 env var instead
so it applies to all uv commands in the step.

Blank window: When the Tauri app starts without the Python backend
running, it showed a completely blank window. Now shows a "Connecting
to backend..." spinner, or an error state with instructions to start
the backend manually.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitea/workflows/build-sidecar.yml

@@ -365,11 +365,14 @@ jobs:
         run: brew install portaudio
 
       - name: Build sidecar (CPU)
+        env:
+          UV_NO_SOURCES: "1"
         run: |
-          # --no-sources bypasses pyproject.toml's [tool.uv.sources] which forces
-          # torch from the CUDA index (no macOS ARM wheels there)
-          # Default PyPI torch includes MPS (Apple Silicon GPU) support
-          uv sync --no-sources
+          # UV_NO_SOURCES bypasses pyproject.toml's [tool.uv.sources] which forces
+          # torch from the CUDA index (no macOS ARM wheels there).
+          # Applies to both uv sync AND uv run (which re-resolves).
+          # Default PyPI torch includes MPS (Apple Silicon GPU) support.
+          uv sync
           uv run pyinstaller local-transcription-headless.spec
 
       - name: Package sidecar (CPU)