Unify action-button states across the UI

- Add UiStyles with explicit normal, hover, pressed, focus and disabled styles and apply it to every action button.
- Disable timeline Previous/Next at the first/last event and keep Close always available.
- Style settings reset-dialog buttons and cover the button-state audit in docs/ui-copy.md.
- Add shared-state and timeline-boundary regression tests.

CHANGELOG.md

@@ -8,6 +8,9 @@ Semantic Versioning.
 
 ### Changed
 
+- All session, settings, radio, Help, review and timeline action buttons now
+  share explicit normal, hover, pressed, focus and disabled styles. Timeline
+  Previous/Next disable at their boundaries.
 - NPC reason answers now use role-specific dialogue contexts, so staff, guests,
   delivery workers, performers and other archetypes answer instead of silently
   deflecting the Reason verb.