Build optimization: pre-built PM3 binaries, ARM64 CI, base image caching

Replace PM3 compile-from-source in pi-gen with pre-built tarball extraction
(saves 43-58 min). Merge stagePM3 into stageDangerousPi as 02-pm3-install
substage, renumber all subsequent substages. Switch CI PM3 build to native
ARM64 runner (ubuntu-24.04-arm64) eliminating QEMU overhead. Add weekly
base-image workflow for pre-baking stages 0-2. Support PM3_TARBALL,
BASE_IMAGE, and APT_PROXY env vars in build-image.sh.

Also includes prior Phase 5 work: theme system, design system integration,
component update system, OS updates, CI build pipeline, and test results.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/backend/main.py

@@ -204,6 +204,17 @@ app.include_router(plugins.router, prefix="/api/plugins", tags=["plugins"], depe
 app.include_router(auth_router, prefix="/api/auth", tags=["auth"])
 app.include_router(ws_router, prefix="/ws", tags=["websocket"])
 
+# Serve theme token CSS from themes/ directory
+theme_paths = [
+    Path(__file__).parent.parent / "frontend" / "themes",   # Development
+    Path("/opt/dangerous-pi/app/frontend/themes"),            # Production
+]
+for tp in theme_paths:
+    if tp.exists() and tp.is_dir():
+        app.mount("/themes", StaticFiles(directory=str(tp)), name="themes")
+        print(f"🎨 Serving themes from: {tp}")
+        break
+
 # Serve frontend static files if build directory exists
 # In production, frontend is built and served from /opt/dangerous-pi/app/frontend/build
 # Priority: 1) check relative to working directory, 2) check absolute production path