chore: remove IS_APPLICATION_ENABLED feature flag (#19081)

## Summary
- Remove the `IS_APPLICATION_ENABLED` feature flag — application
features are now always enabled
- Remove `@RequireFeatureFlag` decorators and `FeatureFlagGuard` from 7
application resolvers (registration, development, manifest, install,
upgrade, marketplace, oauth)
- Remove frontend feature flag checks: settings navigation visibility,
route protection wrapper, side panel widget type gating, and role
permission flag filtering
- Delete the integration test for disabled-flag behavior
- Clean up seed data, test mocks, and generated schema files

packages/twenty-server/src/engine/workspace-manager/dev-seeder/core/utils/seed-feature-flags.util.ts

@@ -30,11 +30,6 @@ export const seedFeatureFlags = async ({
         workspaceId: workspaceId,
         value: true,
       },
-      {
-        key: FeatureFlagKey.IS_APPLICATION_ENABLED,
-        workspaceId: workspaceId,
-        value: true,
-      },
       {
         key: FeatureFlagKey.IS_PUBLIC_DOMAIN_ENABLED,
         workspaceId: workspaceId,