Add favicon: orange broadcast icon matching the app navbar logo

Created frontend/public/favicon.svg using the same IconBroadcast paths from
@tabler/icons-react (v3.44.0) that are used in the AppLayout navbar. The icon is
rendered as white strokes on an orange rounded-square background (#fd7e14,
Mantine orange.5) so it's clearly legible at the 16px favicon size that stroke-only
SVGs collapse to.

The SPA catch-all route in backend/main.py (/{full_path:path}) was swallowing
GET /favicon.svg and returning index.html with Content-Type text/html — browsers
silently ignored the response since it wasn't a valid image. Fixed by registering
an explicit /favicon.svg GET route before the fallback that reads and returns the
file with the correct image/svg+xml content type. The index.html <link rel="icon">
was already pointing at /favicon.svg from the Vite project template; no HTML
changes were needed.

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

Author: AlpineArchitecture

backend/main.py

@@ -86,6 +86,10 @@ async def auth_middleware(request: Request, call_next):
 if STATIC_DIR.exists():
     app.mount("/assets", StaticFiles(directory=str(STATIC_DIR / "assets")), name="assets")
 
+    @app.get("/favicon.svg", include_in_schema=False)
+    async def favicon():
+        return Response(content=(STATIC_DIR / "favicon.svg").read_bytes(), media_type="image/svg+xml")
+
     @app.get("/{full_path:path}", include_in_schema=False)
     async def spa_fallback(full_path: str):
         return HTMLResponse(content=(STATIC_DIR / "index.html").read_text())