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/pages/CreateTaskPage.tsx

@@ -4,7 +4,7 @@ import api from '@/services/api'
 import type { Project, Milestone } from '@/types'
 
 const TASK_TYPES = [
-  { value: 'story', label: 'Story', subtypes: ['improvement', 'refactor'] },  // P9.6: 'feature' removed — must come from propose accept
+  { value: 'story', label: 'Story', subtypes: ['improvement', 'refactor'] },  // P9.6: 'feature' removed — must come from proposal accept
   { value: 'issue', label: 'Issue', subtypes: ['infrastructure', 'performance', 'regression', 'security', 'user_experience', 'defect'] },
   // P7.1: 'task' type removed — defect subtype migrated to issue/defect
   { value: 'test', label: 'Test', subtypes: ['regression', 'security', 'smoke', 'stress'] },