Add SVG favicon matching logo design

Creates a scalable favicon with the same design as the header logo:
dark background with coral-colored circle, grid lines, and center dot.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app.py

@@ -589,6 +589,12 @@ async def reload_data(
 # =============================================================================
 
 
+@app.get("/favicon.svg")
+async def favicon():
+    """Serve favicon."""
+    return FileResponse(settings.frontend_dir / "favicon.svg", media_type="image/svg+xml")
+
+
 @app.get("/robots.txt")
 async def robots_txt():
     """Serve robots.txt for search engine crawlers."""