feat: Add project deletion; untrack CLAUDE.md from git

- Add DELETE /project/<id>/delete route with ownership check, active-run
  guard, filesystem cleanup (shutil.rmtree), and StoryState cascade delete
- Add delete button + confirmation modal to project page header
- Add delete button + per-project confirmation modal to dashboard cards
- Add CLAUDE.md to .gitignore and remove it from git tracking

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -12,6 +12,7 @@ ai_blueprint.md
 plans/
 
 # Claude / Anthropic Artifacts
+CLAUDE.md
 .claude/
 claude.json