[AI] Typescript: low hanging fruit (#7091)

* [AI] Use loot-core workspace in desktop-electron and fix related types

- Add loot-core as workspace dependency in desktop-electron
- Import GlobalPrefsJson from loot-core package in desktop-electron
- Allow null in usersSlice data type (UsersState)
- Add explicit SQL.Database return type to openDatabase in sqlite electron

Made-with: Cursor

* Re-add loot-core as a workspace dependency in desktop-electron

packages/loot-core/src/platform/server/sqlite/index.electron.ts

@@ -100,7 +100,7 @@ function regexp(regex: string, text: string | null) {
   return new RegExp(regex).test(text || '') ? 1 : 0;
 }
 
-export function openDatabase(pathOrBuffer: string | Buffer) {
+export function openDatabase(pathOrBuffer: string | Buffer): SQL.Database {
   const db = new SQL(pathOrBuffer);
   // Define Unicode-aware LOWER, UPPER, and LIKE implementation.
   // This is necessary because better-sqlite3 uses SQLite build without ICU support.