test: Add Tauri command integration coverage

## What
Add integration tests for Tauri command handlers across state, settings, database, audio, Whisper, and LLM boundaries.

## Why
- `.planning/codebase/CONCERNS.md` identifies command integration coverage as a high-priority gap.
- Current tests cover database helpers and some frontend components, but command payloads and app-state wiring can regress independently.
- IPC contract drift can break the UI even when Rust unit tests pass.

## How
- Build a command test harness around `AppStateExt` with an isolated SQLite pool.
- Start with settings, meetings, transcripts, summaries, and LLM URL normalization paths.
- Add audio/Whisper command tests behind mocked or fixture-safe boundaries.
- Keep tests close to real implementations and avoid broad custom doubles.