feat(webui): improve setup flows across chat channels

[chengyongru]

Summary

• expose contract-defined alternative requirements to the WebUI and organize channel fields into account, credentials, connection, mail, access, behavior, and security sections
• make unconfigured channel toggles actionable, focus validation errors, use native radio semantics, support explicit saved-secret removal, and localize the new UI in all supported locales
• add a browser-native WhatsApp QR linking flow with cancellable temporary session storage that only replaces the working database after a successful link
• remove remote channel favicon dependencies in favor of local icons or existing initials/colors

Verification

• uv run --no-sync pytest tests/channels nanobot/channels/whatsapp/tests nanobot/channels/matrix/tests tests/webui/test_settings_system.py -q — 357 passed
• uv run --no-sync basedpyright — 0 errors, 0 warnings
• uv run --no-sync ruff check nanobot/ — passed
• bun run build — passed
• channel-focused Vitest suite — 48 passed
• bunx vitest run src/tests/message-bubble.test.tsx — 36 passed
• real-gateway browser verification — desktop and 390px mobile passed; WhatsApp connect entry visible, Matrix requirement/focus behavior correct, no overflow, and 0 console errors/warnings

Notes

• A live WhatsApp phone was not paired. The browser/gateway QR contract was exercised black-box, while successful database promotion and cancellation preservation are covered with a fake neonize client.
• The full parallel WebUI suite remains load-sensitive in two pre-existing message tooltip hover assertions; those tests pass when their file is run alone, and all channel tests pass.