feat: add userspace Tailscale transport for mobile

[goxofy]

Summary

• Add an optional userspace Tailscale (tsnet) transport for iOS and Android.
• Forward both HTTP and HTTPS backends through a local loopback HTTP listener, preserving HTTPS TLS verification/SNI and backend Host headers.
• Keep the configured display URL separate from the runtime loopback URL across authentication, Hosts loading, and terminal WebSocket connections.
• Add lifecycle-safe login, rehydrate, reconnect, logout, Direct/LAN switching, and forward reuse behavior.
• Add native iOS/Android bridges, Android arm64-v8a build integration, CI native-library generation, and device/build documentation.

Validation

• ✅ npm ci
• ✅ npm run format:check
• ✅ npm run lint (0 errors; existing/new lint warnings remain)
• ✅ npx tsc --noEmit --pretty false
• ✅ npx expo config --type public --json
• ✅ Android network-security, OIDC intent-filter, and widget-contract verification scripts
• ✅ go -C modules/termix-tailscale/native test ./...
• ✅ go -C modules/termix-tailscale/native test -race ./...
• ✅ go -C modules/termix-tailscale/native vet ./...
• ✅ Host archive build and exported-symbol checks
• ✅ iOS arm64 Go archive build and exported-symbol checks

Generated native libraries and app build products are intentionally ignored and are not included in this PR.