feat: update portfolio content - projects, skills, experience, education

- Replace projects with current work (Hongik, SION, A301, tolelog, No-Ill)
- Add project period display
- Add SSAFY embedded track to Education and Experience
- Add No-Ill PM role to Experience
- Update Skills: remove unused (Django, Express.js, Unreal), add actual stack (Redis, WebSocket, JWT, PostgreSQL, LevelDB, CI/CD, Git, Jira)
- Update About section to reflect current status

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

css/style.css

@@ -195,10 +195,15 @@ img { max-width: 100%; display: block; }
   padding: 2px 8px;
   border-radius: 4px;
 }
+.project-card-period {
+  font-size: 0.8rem;
+  color: #999;
+}
 .project-card-desc {
   color: #555;
   font-size: 0.95rem;
   margin-bottom: 14px;
+  margin-top: 8px;
   line-height: 1.6;
 }