Refresh login screen with server list UI and refactor platform config

Redesign login view with server card list, add/edit/delete modals,
and per-server ping status display. Rename ProxyOverrides to ClientConfig,
remove ConfigSystemInterface in favor of direct platform methods
(load_config, load_username, set_default_username, load_server_url),
remove SharedState threading in favor of global STATE, simplify
network_loop and audio setup, update proxy endpoint from /overrides
to /config, and clean up desktop launch configuration.

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

docker/docker-compose.yaml

@@ -20,7 +20,7 @@ services:
   #  volumes:
   #    - ..:/app
   #  environment:
-  #    - MUMBLE_WEB2_PROXY_OVERRIDES_URL=https://localhost:64444/overrides
+  #    - MUMBLE_WEB2_GUI_CONFIG_URL=https://localhost:64444/config
   #  stdin_open: true
   #  tty: true
   #  command: >