feat(config): add configurable VPN MTU with unified resolution across…

[zerox80]

Add client-side VPN MTU configuration across all platforms

Summary

• Add user-configurable VPN MTU (vpn_mtu, range 1280–1360, default 1280) to all client platforms
• QUIC Payload MTU is always derived automatically as vpn_mtu + 80 bytes — never independently settable
• Consolidate per-platform MTU resolution into a single shared function resolve_tun_mtu() with fallback chain: explicit config → VPN_MTU env var → default 1280

Motivation

The server already supports VPN_MTU via .env. However, clients had no way to set the matching TUN MTU — Android was hardcoded to 1280, and Windows/Linux only read the env var. If the server operator changed VPN_MTU (e.g. to 1360 for PPPoE/DSL paths), clients would mismatch and send out-of-spec QUIC packets.

Changes (20 files, +440 / −133)

Shared (shared/)

• lib.rs — Added resolve_tun_mtu(vpn_mtu: Option<u16>) -> u16, MIN_TUN_MTU (1280), MAX_TUN_MTU (1360) constants
• ipc.rs — Added vpn_mtu: Option<u16> field to Config struct (#[serde(default)], backward-compatible)

Android (JNI + Rust + Kotlin)

• jni.rs — Added vpn_mtu: jint parameter to native init() (0 = use default)
• connection.rs — Accepts vpn_mtu: Option<u16>, uses resolve_tun_mtu() instead of hardcoded DEFAULT_TUN_MTU
• NativeLib.kt — Added vpnMtu: Int to external init()
• MaviVpnService.kt — Passes prefs.savedVpnMtu to both NativeLib.init() and VpnService.Builder.setMtu()
• PrefsManager.kt — Added savedVpnMtu: Int SharedPreferences field
• VpnViewModel.kt — Added vpnMtu state flow, updated saveSettings() signature
• SettingsScreen.kt — Added VPN MTU text input field with validation (1280–1360)
• MainActivity.kt — Updated settings callback to pass vpnMtu

Linux CLI

• vpn_core.rs — Removed local read_tun_mtu_from_env(), uses resolve_tun_mtu(config.vpn_mtu)
• main.rs — Added interactive VPN MTU prompt, displays MTU when loading saved config

Windows CLI

• vpn_core/mod.rs — Removed CLIENT_TUN_MTU LazyLock and read_tun_mtu_from_env()
• vpn_core/handshake.rs — Accepts vpn_mtu: Option<u16>, uses resolve_tun_mtu()
• vpn_core/network.rs — Fixed set_adapter_network_config to actually use its tun_mtu parameter (was previously ignored)
• main.rs — Added interactive VPN MTU prompt, displays MTU when loading saved config

GUI (Tauri + Web)

• lib.rs — Added vpn_mtu: Option<u16> to SavedConn struct
• index.html — Added <input type="number" min="1280" max="1360"> in settings form and connection modal
• main.js — Wired vpn_mtu through readSettings(), fillSettings(), openModal(), saveModal(), selectConnection()
• utils.js — Added vpn_mtu to toConfig() helper

Backward Compatibility

• vpn_mtu is Option<u16> with #[serde(default)] — existing configs without the field load fine and fall back to 1280
• The VPN_MTU environment variable still works as a secondary fallback (CLI / daemon use)
• All existing IPC payloads without vpn_mtu continue to work

Testing

• All 201 workspace tests pass (cargo test --workspace)
• IPC bincode roundtrip tests updated with vpn_mtu: None
• GUI SavedConn deserialization test confirms missing field defaults to None
• Clean compilation, zero warnings

MTU Safety — MIN is always 1280

Guard	Location	Enforcement
Backend	backend/src/config.rs	clap value_parser range 1280–1360
Shared constant	shared/src/lib.rs	MIN_TUN_MTU = 1280
Shared resolver	resolve_tun_mtu()	Rejects out-of-range, falls back to 1280
Android	MaviVpnService.kt	in 1280..1360 check, fallback 1280
HTML inputs	gui/src/index.html	min="1280" max="1360"
CLI prompts	Linux/Windows main.rs	>= 1280 && <= 1360 check, fallback None