[Omi] Use local WAL files for audio playback when 'Store Audio on Phone' is enabled

## Problem When "Store Audio on Phone" is enabled, local WAL files are kept on device but **NOT used for playback**. The app always streams audio from the cloud (GCS), even when a local copy exists. **Current behavior:** ```dart // app/lib/widgets/conversation_audio_player_widget.dart final urls = getConversationAudioUrls( conversationId: widget.conversation.id, audioFileIds: audioFileIds, format: 'wav', ); // Always generates: v1/sync/audio/{conversationId}/{audioFileId}?format=wav // Always streams from GCS ``` **Why this is wasteful:** - Downloads same audio multiple times if replayed - Wastes bandwidth and cloud egress costs - Doesn't work offline (even though local file exists) - "Store Audio on Phone" setting has no playback benefit ## Proposed Solution Check for local WAL files first, fall back to cloud streaming if missing. **Playback flow:** ``` Click play → Check if WAL exists locally → If yes: Play from local file (instant, offline-capable) → If no: Stream from