[tkohr]

PR is based on #222, #220 and their underlying backend PRs.

• Loads the schemas and tables of the selected source database (GET /source-databases/{key}/tables) into SourceDatabaseStore and passes them down to app-data-source-db.
• Replaces the schema and table text inputs with dropdowns. The table list follows the selected schema and is disabled until a schema is chosen. Changing the schema clears the table, and changing the table clears the title.
• Refactors the db and ftp source forms from FormBuilder to signals, and documents when to use reactive forms in the Angular skill.

Note: Code was mainly generated with claude.