feat(rdp): connect to gateway-mode RDP routes via server connect_address (Phase C)

[CallMeTechie]

Summary

Android-client half of RDP-over-gateway — completes the 3-repo feature (server v1.63.0 + Pro Windows client already shipped). Lets the Android client launch FreeRDP sessions to access_mode='gateway' routes (targets that are NOT WireGuard peers, reached through a Home Gateway via the public <server>:<listen_port> endpoint).

• RdpRoute DTO gains connectAddress/connectPort (@SerializedName, nullable defaults — additive, backward-compatible with older servers).
• RdpConnectionParams.fromRoute uses connectAddress ?: host / connectPort ?: port as the FreeRDP target.
• VPN requirement is exempted for gateway routes via a pure, testable RdpManager.requiresVpn(accessMode) (case-insensitive; null/other → still requires VPN).
• connect() locals (external-launch path, session monitor, error strings) now use the effective connect endpoint for consistency with the embedded path.

End-to-end (embedded/production path): server connect_address → DTO → fromRoute → RdpBookmarkBuilder.hostname → FreeRDP connects to the public endpoint.

Test Plan

• CI: ./gradlew build + unit tests pass (this is the compile gate — not built locally)
• New unit tests: RdpConnectionParamsTest (gateway uses connectAddress; null fallback) + RdpManagerVpnGateTest (gateway exempt; internal/external/both/vpn/null require VPN; case-insensitive)
• Manual: gateway RDP route, VPN OFF → Android connects to <server>:<listen_port>; credentials applied
• Manual: internal route still requires VPN and connects to the peer IP as before

Known/non-blocking follow-ups: RdpHostCard still displays the LAN host for gateway routes (server doesn't set externalHostname for gateway) — cosmetic, pre-existing; consider showing connectAddress / a "via gateway" label. RdpManager.connect() full control flow remains unit-test-free (requiresVpn + fromRoute are covered).