Finalize Dexie-to-SQLite migration prep: docs, circular dep removal, SQL helpers, tests

- Removed all vestigial Dexie/USE_DEXIE_DB references from code and docs
- Centralized DB logic in PlatformServiceMixin; resolved logger/databaseUtil circular dependency
- Modularized SQL helpers (`$generateInsertStatement`, `$generateUpdateStatement`) and added unit tests
- Created/updated migration tracking docs and helper script for cross-machine progress
- Confirmed all lint/type checks and tests pass; ready for systematic file migration

src/db/tables/accounts.ts

@@ -45,7 +45,7 @@ export type Account = {
   publicKeyHex: string;
 };
 
-// TODO: When finished with USE_DEXIE_DB, move these fields to Account and move identity and mnemonic here.
+// TODO: When finished with migration, move these fields to Account and move identity and mnemonic here.
 export type AccountEncrypted = Account & {
   identityEncrBase64: string;
   mnemonicEncrBase64: string;