Wire Proton VPN into provider UI

[kasnder]

Summary

• add Proton as a VPN provider in onboarding and the provider switcher
• support Proton SRP login, TOTP, country discovery, profile generation, activation, and sign-out
• generate the Ed25519 identity required by Proton and convert it to the matching X25519 WireGuard private key
• persist the session and key material so country selection survives UI recreation; passwords are zeroed by the auth client and never stored

Validation

• cargo test (22 passed)
• ./gradlew :app:testGithubDebugUnitTest --tests net.kollnig.missioncontrol.wg.proton.* (7 passed)
• ./gradlew assembleGithubDebug
• installed with adb install -r -d on Pixel 8 44230DLJH00128

Known validation boundary

• real Proton credentials were not available to exercise the live API; mocked auth, TOTP, certificate, logical-server, and profile flows pass
• Android lint still reports six pre-existing translation format errors; none point to this diff

Depends on #720.

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