feat(FN-1634): promote mailbox to first-class navigation view

- Add MailboxView as a full-page navigation view replacing the modal-based approach
- Migrate MessageStore from filesystem to SQLite backend for message persistence
- Implement conversation grouping for inbox display with unread badge state
- Remove modal plumbing (useModalManager, AppModals exports) and simplify App.tsx
- Add evictStaleProcessing() to TriageProcessor for self-healing hung triage sessions
- Add comprehensive MailboxView tests and Header mailbox tests
- Update README documentation with MailboxView features
- Add CSS styles for MailboxView component
- Fix MobileNavBar tests for mailbox tab visibility

packages/engine/src/runtimes/in-process-runtime.ts

@@ -479,6 +479,7 @@ export class InProcessRuntime
         getExecutingTaskIds: () => this.executor.getExecutingTaskIds(),
         recoverApprovedTriageTask: (task) => this.triageProcessor?.recoverApprovedTask(task) ?? Promise.resolve(false),
         getSpecifyingTaskIds: () => this.triageProcessor?.getProcessingTaskIds() ?? new Set<string>(),
+        evictStaleTriageProcessing: () => this.triageProcessor?.evictStaleProcessing() ?? new Set<string>(),
       });
       this.selfHealingManager.start();
       this.stuckTaskDetector.start();