Let AnkiDroid players choose their study companion

[AldanaBRZ]

Adds the Aki, Ankilope, and no-companion account choice to AnkiDroid. The picker lives under AnkiQuest → Account, syncs through the companion API from float3/ankiquest#55, and remembers the last server choice per account for offline display. The home screen, deck heatmap, widget, settings icon, and decorative notification art follow the choice; study rules and notifications remain unchanged.

The change also updates settings validation for the recently split Account, Study, and Notifications menus, including the new companion setting.

Validation: fullDebug Kotlin compilation and main/test Kotlin style checks passed locally; emulator checks passed on the first CI run. The account-scope test is included. The local unit-test task is blocked by a Gradle cache access error, so CI runs the complete unit suite.

[github-actions]

Snapshot diff report vs ankiquest. Open screenshot-diff for diffs.

• AllActivitiesScreenshotTest: 2 changes
• PreferencesScreenshotTest: 1 change
• ReviewRemindersScreenshotTest: 1 change

All 4 changed screenshots

AllActivitiesScreenshotTest

• AnkiquestHomeActivity_compare.png
• AnkiquestHomeActivity_edgeToEdge_compare.png

PreferencesScreenshotTest

• AnkiquestAccountSettingsFragment_compare.png

ReviewRemindersScreenshotTest

• studyOptionsFragmentHost_troubleshooting_compare.png