Fix favicon not showing issue

- Use url_for() for all favicon and icon references in layout.html
- Add specific routes for favicon files to ensure proper serving
- Create dynamic webmanifest route with correct icon paths
- Import send_from_directory for serving static files
- This ensures favicons work correctly in all deployment scenarios

static/site.webmanifest

@@ -0,0 +1 @@
+{"name":"","short_name":"","icons":[{"src":"/android-chrome-192x192.png","sizes":"192x192","type":"image/png"},{"src":"/android-chrome-512x512.png","sizes":"512x512","type":"image/png"}],"theme_color":"#ffffff","background_color":"#ffffff","display":"standalone"}