FE-PR-001: Rename Propose -> Proposal across frontend

- Rename ProposesPage -> ProposalsPage, ProposeDetailPage -> ProposalDetailPage
- Update Propose type to Proposal (keep Propose as deprecated alias)
- Add GeneratedTask type for accept results
- Switch API calls from /proposes to /proposals (canonical)
- Update sidebar label: Proposes -> Proposals
- Update routes: /proposals (+ legacy /proposes compat)
- Update all UI text: Propose -> Proposal
- Remove feat_task_id display, add generated_tasks section
- Clean up propose references in comments

src/components/Sidebar.tsx

@@ -32,7 +32,7 @@ export default function Sidebar({ user, onLogout }: Props) {
   const links = user ? [
     { to: '/', icon: '📊', label: 'Dashboard' },
     { to: '/projects', icon: '📁', label: 'Projects' },
-    { to: '/proposes', icon: '💡', label: 'Proposes' },
+    { to: '/proposals', icon: '💡', label: 'Proposals' },
     { to: '/notifications', icon: '🔔', label: 'Notifications' + (unreadCount > 0 ? ' (' + unreadCount + ')' : '') },
     { to: '/monitor', icon: '📡', label: 'Monitor' },
     ...(user.is_admin ? [