feat(app): add device button action switch

[arhxam]

What changed and why

Add a default-on, persisted master switch for Omi device button actions. Interactive onboarding still receives the button event first, while normal single-, double-, and long-press actions return before side effects when the user disables the switch.

Closes #4458

Product invariants affected

none

How it was verified

• flutter test test/unit/device_button_policy_test.dart test/widgets/device_settings_double_tap_test.dart — all 9 preference, policy, persistence, and settings-widget tests passed.
• flutter gen-l10n — generated all 48 supported locale implementations with zero untranslated messages.
• bash scripts/analyze_ratchet.sh — analyzer ratchet passed.
• The physical button path was not exercised because no paired Omi device is available locally.

Tests

• Added default-on, disabled, and persistence policy coverage.
• Added widget coverage for toggling and reloading the setting.
• Regenerated and validated every supported locale.
• Physical-device button presses (hardware unavailable).

Failure-Class: none

[Git-on-my-level]

Thanks @arhxam - this is a clean, well-scoped implementation of #4458, and the default-on choice preserves today's behavior for existing users.

What I verified:

• app/lib/services/capture/capture_controller.dart - the gate sits inside the single onButtonReceived handler (the only BLE button listener registration; every device connector funnels through it), after the interactive-onboarding interception and before the double-tap, single-tap, and legacy long-press branches (button states 1/2/3/5), so one check covers every user-configured action with no bypass path.
• app/lib/backend/preferences.dart - deviceButtonEnabled defaults to true, so existing users are unaffected until they opt out.
• app/lib/services/capture/device_button_policy.dart - small policy seam that keeps the gate unit-testable; app/test/unit/device_button_policy_test.dart covers default-on, disabled, and reload persistence, and app/test/widgets/device_settings_double_tap_test.dart exercises the actual switch widget plus persistence across a reload.
• l10n - the two new strings carry @ descriptions in app_en.arb, all supported locales are populated, and the regenerated app_localizations*.dart files match (Generated Files check is green).
• CI: Dart Analyze & Tests, Formatting, Android Compile Smoke, and PR Metadata Preflight all pass.

Two non-blocking notes for the maintainer:

1. The switch is a single global preference but lives on a per-device settings page - worth deciding whether v1 should stay global (matches the issue's ask) or become per-device.
2. During interactive onboarding step 1, the single-tap fall-through reaches the new gate, so a user who has disabled button actions will not get the voice-command demo tap during re-onboarding (the onboarding provider still receives the event first). Probably the right trade-off, just flagging the interaction.

Nice work shipping tests alongside the feature. Product direction decision on the switch scope (global vs per-device) needs maintainer sign-off before merge.

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