Add opt-out for unassigned-gesture toasts

[NicolasBonduel]

📝 Description

Adds a new Settings toggle, "Show Toast for Unassigned Gestures", placed directly below the existing "Show Toast for Entity Trigger" row in the Advanced Settings card. It is on by default, matching current behavior exactly, so no existing behavior changes unless a user explicitly opts out.

Changes:

• AppPrefs.kt: new pref_show_toast_on_unassigned_gesture key with getter/setter, defaulting to true.
• QuickBarService.kt: the three showToast("No … action assigned") call sites (single-press, double-press, long-press) now route through a new hintUnassigned() helper that checks the pref before toasting.
• fragment_settings.xml / SettingsFragment.kt: new switch row wired the same way as the existing toast toggle.

🔍 Related Issue

Fixes #63

💡 Motivation and Context

Reported in #63: mapping a key (e.g. Home) with Single Press Action = None and a double or long-press action assigned causes "No single-press action assigned" to toast on every normal press. This happens because that toast lives inside the double-press branch of handleTriggerGestures(), which fires on every short tap regardless of whether the single-press slot was intentionally left as None. There was previously no way to suppress it. This PR adds an explicit opt-out rather than changing the default, so existing users relying on the toast for other unmapped gestures see no change unless they choose to turn it off.

📺 Screenshots / Video (if applicable)

N/A — this is a single new switch row, structurally identical to the existing "Show Toast for Entity Trigger" row directly above it.

🧪 How Has This Been Tested?

• ./gradlew assembleDebug builds cleanly with no new warnings.
• Installed on a physical Google TV Streamer, Android 14 (the device from the original report), via adb.
• Verified: mapping Home with single = None / double = <QuickBar>, toggling the new setting off silences the toast on a normal Home press while the double-press action still fires; toggling it back on restores the toast.

📚 Documentation Updates

• No documentation changes required.

✅ Checklist

• My code follows the code style of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• My changes generate no new warnings or build errors.

[NicolasBonduel]

Note on the default: isShowToastOnUnassignedGestureEnabled defaults to true when the pref hasn't been written yet. That's intentional: it preserves current behavior for existing users exactly as-is.