feat(plugins): support enum settings

[ODevStudio]

Summary

What changed, and why?

• Render plugin settings declared as enum with a select control.
• Normalize only stale stored enum values before saving, using a valid manifest default or an explicit deletion tombstone.
• Define enum values as a JSON array of strings and share one parser between manifest loading, service validation, and the settings UI.
• Reject non-null enum settings outside the manifest's declared values in PluginLoaderService, including non-UI callers.

Linked Issue

Fixes #255

Verification

How did you verify the change? Include relevant tests and any manual or hardware testing.

• flutter test --no-pub test/plugin_loader_service_appstore_test.dart test/plugins_settings_view_appstore_test.dart (42 passed)
• flutter analyze --no-pub (no issues)
• flutter test --no-pub (3,220 passed, 1 skipped; the known Windows CRLF-sensitive AsyncAPI schema assertion failed, while CI/Linux uses LF)

Impact

Note any user-visible behavior, compatibility, migration, API/spec, documentation, or security impact. Write None if there is none.

• Plugin manifests can expose enum settings as a fixed-choice selector. Enum manifests use JSON string arrays, and invalid values are rejected at the service boundary. No migration or security impact.

Contributor Responsibility

AI-assisted development is allowed. The submitter remains responsible for the submitted work.

• I have reviewed and understand all changes in this PR and take responsibility for their correctness, security, behavior, licensing, and provenance, including any AI-assisted or AI-generated work.

[tadelv]

Two changes requested before merge:

1. enum validation currently exists only in PluginsSettingsView. PluginLoaderService._validateSettings() only checks whether a key exists, so callers outside this UI can still persist arbitrary values for an enum setting. Since this PR introduces enum as a plugin setting type, please move/share enum-value parsing at the settings-schema/service layer and reject non-null values outside the declared enum. The UI should consume the same parsed values so the two paths cannot drift.

2. Since this defines the manifest contract, please use a JSON array for values (for example "values": ["Light", "Medium", "Dark"]) rather than a pipe-delimited string. The array is structurally valid JSON, avoids delimiter/escaping concerns, and is substantially easier to validate consistently.

The rest of the implementation looks good, including stale-value handling and preserving untouched defaults.