feat: add network proxy settings

[power721]

Summary

• add persisted advanced proxy settings with manual modes, editable bypass rules, and a dedicated Advanced Settings tab
• add a centralized proxy decision layer and wire it through API/metadata clients, poster loading, playback parsers, danmaku, plugin loading, HLS upstream fetches, compat spider requests, and yt-dlp
• add focused tests for storage, proxy rules, UI validation, runtime fetchers, and yt-dlp proxy argument generation

Test Plan

• uv run python -m py_compile src/atv_player/app.py src/atv_player/danmaku/direct_parse.py src/atv_player/danmaku/service.py src/atv_player/playback_parsers.py src/atv_player/player/ytdlp_runtime.py src/atv_player/plugins/loader.py src/atv_player/plugins/compat/base/spider.py src/atv_player/proxy/segment.py src/atv_player/ui/poster_loader.py src/atv_player/yt_dlp_service.py
• uv run pytest tests/test_storage.py -k network_proxy -q
• uv run pytest tests/test_network_proxy.py tests/test_main_window_ui.py -k "network_proxy or advanced_settings_dialog" tests/test_api_client.py tests/test_metadata_tmdb_client.py tests/test_local_douban_client.py tests/test_direct_parse_danmaku.py -q
• uv run pytest tests/test_poster_loader.py tests/test_playback_parsers.py tests/test_spider_plugin_loader.py tests/test_hls_proxy_segment.py tests/test_yt_dlp_service.py tests/test_app.py -k "proxy or AppCoordinator or build_application" -q