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>

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