CATROID-1665 Add Settings toggle to enable/disable the AI Tutor

[harissabil]

Summary

Adds a top-level "AI Tutor extension" checkbox to Settings so users can turn the AI Tutor on or off.

• New setting_ai_tutor CheckBoxPreference (off by default) in preferences.xml, gated by the BuildConfig.FEATURE_AI_ASSIST_ENABLED build flag (entry removed when the feature isn't shipped).
• SettingsFragment.isAiTutorSharedPreferenceEnabled() helper for runtime reads.
• The AI Assist button now shows only when both the build flag and the user preference are enabled; re-evaluated in onResume so toggling takes effect on return.
• Flips FEATURE_AI_ASSIST_ENABLED to true so the toggle is visible.
• Extends SettingsFragmentTest.basicSettingsTest() to cover the new toggle.

Screenshot

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[reichli]

Looks good - thank you!