refactor: revert to JSON persistence for 2.2.x release (remove SQLite/sql.js)

Remove SQLite (sql.js/WASM) storage layer and restore Obsidian native JSON persistence via loadData()/saveData(). SQLite caused sync issues: full binary rewrite on every save, no merge capability, and sync conflicts leading to corruption. SQLite work continues on feat/sqlite-migration for a future 3.0.0 release.

Removed:
- src/services/database.ts (DatabaseService)
- src/services/migrator.ts (MigrationService)
- src/types/sql.js.d.ts (sql.js type definitions)
- sql.js dependency from package.json
- SQLite-related commands, methods, and UI sections

Author: amatya-aditya