introduce VpnConfig trait, refactor connect_vpn signature

[cachebag]

part of #288

current VpnCredentials has wireguard-specific fields baked in. introduce a trait-based abstraction so openvpn (and future protocols) don't bloat the same struct.

changes

add VpnConfig trait in api/models/vpn.rs:

pub trait VpnConfig: Send + Sync + std::fmt::Debug {
    fn vpn_type(&self) -> VpnType;
    fn name(&self) -> &str;
    fn gateway(&self) -> &str;
    fn dns(&self) -> Option<&[String]>;
    fn mtu(&self) -> Option<u32>;
    fn uuid(&self) -> Option<uuid::Uuid>;
}

add WireGuardConfig struct as the new preferred wireguard config type, implementing VpnConfig.

impl VpnConfig for VpnCredentials for backward compat — deprecate VpnCredentials with a note pointing to WireGuardConfig.

change connect_vpn(&self, config: impl VpnConfig) in network_manager.rs. non-breaking since VpnCredentials implements the trait and callers use type inference.

files

• api/models/vpn.rs — trait, WireGuardConfig, deprecation
• api/network_manager.rs — signature change
• lib.rs — re-exports

[cachebag]

Finished in #303