feat(database): integrate wizard into settings page, improve accessibility and test coverage

- Add "DB Wizard" link button to settings page header
- Add help_link to database_url SETTING_METADATA pointing to /database-wizard
- Add help_link rendering in settings template for any setting with a help_link
- Fix SQLite whitespace path handling in build_connection_string
- Add dark mode CSS overrides for wizard template
- Add aria-describedby for all form inputs with help text
- Add prefers-reduced-motion media query for smooth scrolling
- Expand test coverage: 106 tests (up from 49)
  - db_wizard.py: 100% coverage
  - db_wizard view: 100% coverage
  - database.py API: 97.37% coverage
  - db_migrate.py: 96.60% coverage

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

app/utils/db_wizard.py

@@ -82,7 +82,9 @@ def build_connection_string(
         ValueError: If required fields are missing for the chosen backend.
     """
     if backend == "sqlite":
-        path = sqlite_path.strip() if sqlite_path else "./app/database.db"
+        path = sqlite_path.strip() if sqlite_path else ""
+        if not path:
+            path = "./app/database.db"
         return f"sqlite:///{path}"
 
     # Resolve driver prefix