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/dashboard/app/components/ModelOnboardingModal.tsx

@@ -880,7 +880,6 @@ import { ActivityLogModal } from "./ActivityLogModal";
 import { GitManagerModal } from "./GitManagerModal";
 import { WorkflowStepManager } from "./WorkflowStepManager";
 import { AgentListModal } from "./AgentListModal";
-import { MailboxModal } from "./MailboxModal";
 import { SetupWizardModal } from "./SetupWizardModal";
 import { ToastContainer } from "./ToastContainer";
 
@@ -1106,14 +1105,6 @@ export function AppModals({
         projectId={projectId}
       />
 
-      <MailboxModal
-        isOpen={modalManager.mailboxOpen}
-        onClose={modalManager.closeMailbox}
-        projectId={projectId}
-        addToast={addToast}
-        agents={modalManager.mailboxAgents}
-      />
-
       {modalManager.setupWizardOpen && (
         <SetupWizardModal
           onProjectRegistered={projectActions.handleSetupComplete}