feat: integrate vectordb for note embeddings

- Added `vectordb` as a dependency in `package.json`.
- Implemented `embedText` function in `src/main/ai/embeddings.ts` to handle text embeddings using GitHub Copilot OAuth token or OpenAI token.
- Created `vectordb.ts` for managing LanceDB connection and embedding notes with upsert strategy.
- Updated `index.ts` to initialize vector database and migrate existing notes on app ready.
- Modified `router/index.ts` to fire-and-forget embedding calls on note creation and updates.
- Enhanced `progress.txt` with detailed implementation notes and learnings regarding the integration.

package.json

@@ -79,6 +79,7 @@
     "remark-gfm": "^4.0.1",
     "tailwind-merge": "^3.5.0",
     "tw-animate-css": "^1.4.0",
+    "vectordb": "^0.21.2",
     "zod": "^4.3.6"
   }
 }