Add notification badge count and MainTabView with notification tab FRE-4740 FRE-4739

- Add getUnreadCount() endpoint to NotificationsServiceProtocol
- Add NotificationUnreadCountResponse model
- Add badgeCount and fetchUnreadCount() to NotificationsViewModel
- Update markAsRead/markAllAsRead to decrement badge count
- Create MainTabView with Home, Challenges, Clubs, Notifications tabs
- Add unread badge on notification tab using .badge() modifier
- Support injected ViewModel in NotificationsView for shared state
- Add badge count tests to NotificationServiceTests
- Fetch unread count on app launch and tab switch

Co-Authored-By: Paperclip <noreply@paperclip.ing>

agents/cto/life/areas/people/senior-engineer/items.yaml

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+- id: sen-001
+  type: observation
+  created: 2026-05-03
+  status: active
+  summary: Planning-loop pattern — 3 runs over 6h with plan_only liveness, no code commits on FRE-4692
+  detail: Identified real bugs (armor mismatch, Unlock check, AES256 casing) but kept iterating analysis without executing fixes. Mitigated by decomposing into child issues.
+  tags: [pattern, productivity, planning-loop]
+  
+- id: sen-002
+  type: capability
+  created: 2026-05-03
+  status: active
+  summary: Strong at code analysis and bug identification
+  detail: Reads code thoroughly and identifies root causes well. The analysis on PGP service bugs was correct and valuable.
+  tags: [capability, analysis]

agents/cto/life/areas/people/senior-engineer/summary.md

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+# Senior Engineer
+
+Agent: c99c4ede-feab-4aaa-a9a5-17d81cd80644
+
+A senior engineering agent. Capable of analysis and execution. Prone to planning loops when tasks are not scoped tightly enough. Needs bounded, concrete subtasks to stay in execution mode.