refactor: rename project Lyria Studio → Kithara Studio

- App name, UI text, page titles → Kithara Studio
- Python module: lyria_client.py → kithara_client.py
- Tailwind color classes: lyria-* → kithara-*
- CSS custom properties: --lyria-* → --kithara-*
- Database: lyria.db → kithara.db
- Package name: kithara-studio-frontend
- localStorage key: kithara-saved-searches
- Add project logo (lyre icon, black on transparent, invert via CSS)

Preserved as Google's property:
- LYRIA_API_KEY, LYRIA_BASE_URL, LYRIA_DEFAULT_MODEL env vars
- LYRIA_MODELS dict and model IDs (lyria-3-clip-preview, lyria-3-pro-preview)
- Google Lyria API references in error messages and docstrings

backend/tests/conftest.py

@@ -1,4 +1,4 @@
-"""Shared test fixtures for Lyria Studio backend."""
+"""Shared test fixtures for Kithara Studio backend."""
 
 import asyncio
 import os
@@ -39,7 +39,7 @@ async def app_client(tmp_path):
     waveforms_dir.mkdir()
     artwork_dir = data_dir / "artwork"
     artwork_dir.mkdir()
-    test_db = data_dir / "lyria.db"
+    test_db = data_dir / "kithara.db"
 
     # Set env vars BEFORE import so module-level Path() calls use test dirs
     # (image_gen.py reads ARTWORK_DIR from env, main.py uses __file__-relative paths)