mobile: add accessibility labels to voice preview controls

Goal

Make each Voice Gallery sample card announce a useful, localized action and voice identity to screen-reader users.

Scope

• Edit apps/mobile/src/screens/VoiceGalleryScreen.tsx.
• Add an explicit accessibilityLabel to the Pressable that plays an individual voice sample.
• Build the label from existing localized STRINGS.voiceGallery.play / playing text plus the voice's characterName and styleName.
• Reflect the current active state in the announced text; an appropriate accessibilityState may be added if it matches React Native semantics.
• Keep playback behavior, layout, filters, API calls, and translation dictionaries unchanged.
• Do not combine this with the broader accessibility-regression work tracked in #48.

Acceptance Criteria

• An idle voice card announces the play action and identifies the character/style.
• The currently playing card announces its playing state and the same voice identity.
• Labels are derived from existing localized strings rather than new hardcoded Korean or Japanese copy.
• Existing Voice Gallery behavior and TypeScript checks remain passing.
• The PR body includes Closes #14 and names the checks run.

Suggested Verification

cd apps/mobile
npm run verify
cd ../..
python3 scripts/check_public_tree.py

Codespaces can run the checks without changing your local environment: https://duct-tape2.github.io/ai-language-partner/community/CODESPACES_FIRST_PR.html