Part of the Godot→Flutter migration (#79). Sibling of the desktop self-update work.

Current state

On Android the updater only links to the .apk asset (platformAssetUrl() → .apk, update_controller.dart:194-195); the user must manually download and tap it. In-place install is deferred (update_controller.dart:75-78).

What's needed

• Download the matched .apk (daccord-android.apk) to app storage with progress UI.
• Launch the system package installer via an ACTION_VIEW/ACTION_INSTALL_PACKAGE intent over a FileProvider URI.
• Request/justify REQUEST_INSTALL_PACKAGES ("install unknown apps") permission and guide the user through enabling it.
• Handle the post-install relaunch.

Notes

• Play Store builds can't sideload — this path is for the direct-download distribution only. If a Play listing is added later, that uses in-app updates instead (separate concern).
• Reference: the Godot client hands the APK to OS.shell_open (../daccord/scripts/autoload/updater.gd _apply_android_update).

Acceptance

Tapping "Update" downloads the APK and opens the installer in one flow, with the permission prompt handled gracefully.

Migrated from DaccordProject/daccord-app#81