refactor(connections): compose connection forms from reusable network, credentials, and SSL sections #1364

Summary

Currently, five separate connection forms exist across the codebase:

• postgresql_connection_form.dart (863 lines)
• mysql_connection_form.dart (650+ lines)
• mongodb_connection_form.dart (768 lines)
• sqlite_connection_form.dart (500+ lines)
• extension_connection_form.dart

These forms duplicate 60–70% of their UI and validation logic:

1. Host, port, and socket fields with identical integer range validations (1–65535).
2. Username and password fields with identical show-password eye toggle buttons (as identified in epic(ui): buttons in the Querya UI kit: one control scale, our own button family, no shadcn buttons #1331: "the show-password eye button is pasted into four forms").
3. Database / schema name inputs, SSH Tunnel sections, and SSL certificate fields.
4. Test connection and save action handling (partially targeted by refactor(ui): migrate dialogs and forms to QueryaButton and QueryaDialogActions #1338 for footers).

Proposed Solution

1. Create reusable composite form sections in lib/features/connections/forms/:
   • ConnectionHostPortSection: Host, port, and unix socket inputs with unified validation.
   • ConnectionAuthSection: Username, password with shared password-visibility toggle from Querya UI Kit.
   • ConnectionDatabaseSection: Database name, schema, search path.
2. Refactor PostgreSQL, MySQL, and MongoDB connection forms to compose these shared sections rather than duplicating input trees.

Acceptance Criteria

• Shared network and credentials sections are used across connection forms.
• Password visibility toggle logic is unified into a single reusable component.
• Existing form validation, saving, and secret storage behavior remains unchanged.