feat: implement browser-only remote sharing

[rrbe]

Summary

• implement opt-in browser-only remote sharing through a configured HTTPS origin and user-owned frp SSH gateway
• launch /usr/bin/ssh reverse tunnels with strict host-key verification, reconnect, expiry, and network rebind handling
• enforce 128-bit tokens, remote read-only access, Secure cookies, and trusted forwarded-client handling
• add remote settings/share UI, architecture documentation, unit tests, and a headless smoke test

Validation

• swift test — 51 tests passed
• all tools/smoke-*.sh scripts and node tools/smoke-md-link-sanitize.cjs
• ./build.sh release assembly
• dependency gate with otool -L
• codesign --verify --deep --strict dist/LocalShare.app
• bundled release app headless smoke test

Notes

Public Nginx/frp deployment and end-to-end internet acceptance remain environment-specific; this PR contains the app-side implementation and local validation.