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>

common/src/lib.rs

@@ -1,7 +1,7 @@
 use serde::{Deserialize, Serialize};
 
 #[derive(Debug, Clone, Deserialize, Serialize, Default)]
-pub struct ProxyOverrides {
+pub struct ClientConfig {
     pub proxy_url: Option<String>,
     pub cert_hash: Option<Vec<u8>>,
     pub any_server: bool,