refactor(plugins)!: remove registry themes, Browse, counts and toggles

[datlechin]

Stacked on #3377. Merge after #3380, which gives Parquet its install path in the Export dialog.

Why

Settings > Plugins carried a catalog the app no longer needs. Drivers install from the connection picker, Parquet from the Export dialog, and updates stay on the Installed rows. The theme registry never published a theme, download counts were the app's only api.github.com call, and the on/off switch saved nothing: lazy plugins cost nothing until used, and a switched-off type still showed in the picker and then failed at connect.

Change

• Removed the Browse tab, the registry plugin detail view and download counts. Settings > Plugins is the installed list.
• Removed theme install from the registry: the Registry theme section, Uninstall, the registry theme folder and the theme category.
• Removed the per-plugin switch, the disabled-plugins setting, the Enable Plugin recovery action and its error.
• A registry entry with an unknown category now decodes as other instead of failing the whole manifest.

Verified

172 cases across the plugin, registry, connection recovery and theme suites pass. New tests: unknown category decoding and installed types always offered. The UI test for Enable Plugin is deleted with the feature.

Not in this PR

• No migration: the registry never shipped a theme, and the com.TablePro.disabledPlugins key stays on disk unread.