Android: back button navigates webview history

[naltatis]

fixes #257

Android's hardware/gesture back button closed the app from anywhere. The evcc web UI pushes history entries for routes and modals, so the native side can simply walk the WebView history instead of finishing the activity.

• Back button steps back through the WebView history (routes and modals); at the root it keeps the default behavior and leaves the app.
• React Navigation modals (server switcher etc.) are unaffected, they already handle back natively.
• iOS unchanged.
• Also documents the commit/PR prefix convention (platform or app area) in AGENTS.md.