feat: inbox actor tracking, issue detail extraction, UI polish

- Add actor_type/actor_id to inbox items for proper attribution
- Extract issue detail into features/issues/components/issue-detail.tsx
- Inbox page and store updates for actor-based notifications
- Sidebar, layout, and actor-avatar refinements

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/pkg/db/generated/models.go

@@ -128,6 +128,8 @@ type InboxItem struct {
 	Read          bool               `json:"read"`
 	Archived      bool               `json:"archived"`
 	CreatedAt     pgtype.Timestamptz `json:"created_at"`
+	ActorType     pgtype.Text        `json:"actor_type"`
+	ActorID       pgtype.UUID        `json:"actor_id"`
 }
 
 type Issue struct {