Add unit test suite

[Lucasmind]

Priority: MEDIUM

Problem

The project has zero tests. This creates high regression risk when refactoring core pipeline logic like thinking tag detection, sentence splitting, and TTS queue ordering.

Suggested Approach

Add a Jest test suite with focus on the most critical paths:

Test Structure

frontend/
├── src/
└── tests/
    ├── unit/
    │   ├── Orchestrator.test.js    # Thinking detection, sentence splitting, state transitions
    │   ├── LLMClient.test.js       # SSE parsing, abort handling, error types
    │   ├── TTSManager.test.js      # Queue ordering, failure handling, PCM decoding
    │   ├── ConversationManager.test.js  # History trimming, emotion tag parsing
    │   ├── EmotionController.test.js    # Emotion mapping, decay timing
    │   └── STTManager.test.js      # VAD integration, transcription handling
    ├── integration/
    │   └── pipeline.test.js        # End-to-end pipeline with mocked services
    └── fixtures/
        └── mock-responses.js       # Mock SSE streams, TTS responses

Critical Test Cases

1. Thinking tag detection — <think> present, absent, malformed (<think without close)
2. Sentence splitting — Standard punctuation, ellipsis, abbreviations, quotes, Unicode
3. Emotion tag parsing — Known emotions, unknown emotions (should default to neutral), no tag
4. TTS queue ordering — Sentences must play in order, queue clear during processing
5. Retry logic — Verify exponential backoff timing, max retries respected
6. State machine — No invalid transitions, barge-in interrupts correctly

Files Affected

• frontend/package.json (add jest dependency + test script)
• New test files as described above