Three home-screen issues from the first Tauri Android APK:
1. Alias was empty (no seed-derived name).
Port the adjective+noun word lists from the old Kotlin SettingsRepository
into a `derive_alias()` helper that maps the first 4 bytes of the seed to
indices in those lists. Same seed → same alias forever, different seeds →
effectively random aliases — so reinstalls keep the user's identity AND
the friendly name they're used to.
2. Build identity was invisible — couldn't tell which APK was actually
installed (this caused us a lot of grief on the Kotlin app).
build.rs now captures `git rev-parse --short HEAD` and emits it as
`WZP_GIT_HASH`, exposed via a new `get_app_info` command. The frontend
stamps `build <hash> • <alias>` under the fingerprint on the home screen.
3. Register on relay failed with `Permission denied (os error 13)`.
Root cause: I hardcoded `/data/data/com.wzp.phone/files/.wzp` as the
identity dir, but the Tauri Android package id is `com.wzp.desktop` —
so the app was trying to write into another app's data directory and
getting EACCES at the filesystem layer. Fix: resolve the data dir from
Tauri's `path().app_data_dir()` API in the `setup()` callback and stash
it in a `OnceLock<PathBuf>`. Works on Android, macOS, Linux, Windows
without any cfg gymnastics.
Also: `get_app_info` returns the resolved `data_dir` so we can debug
storage issues from the UI (it's set as the build-hash element's title).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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