Make v1 mobile-only and split gateway agents

[botlong]

Summary

• Drop Flutter Web support for v1 and remove app-level gateway auth settings/UI.
• Split gateway agent adapters into per-agent modules with shared helper modules and boundary tests.
• Refresh docs for mobile/iOS-only scope, trusted LAN/Tailscale access, and the new adapter layout.

Test Plan

• npm test --prefix gateway
• Docker Flutter test: flutter pub get && flutter test
• Docker Flutter analyze: flutter pub get && flutter analyze
• Verified web directory removal and stale Web/token documentation scans.