feat: add version display to frontend and /api/version endpoint (v1.2.5)

- Add GET /api/version endpoint that reads VERSION file
- Frontend fetches version on init and displays it as a badge in the header
- Add version-badge CSS styling
- Update docker-compose.yml comment to v1.2.5

backend/frontend/style.css

@@ -433,6 +433,20 @@ input {
   color: var(--muted);
 }
 
+.version-badge {
+  display: inline-block;
+  margin-left: 8px;
+  padding: 2px 8px;
+  border-radius: 999px;
+  background: rgba(125, 211, 252, 0.15);
+  border: 1px solid rgba(125, 211, 252, 0.3);
+  color: var(--accent-strong);
+  font-size: 11px;
+  font-weight: 600;
+  letter-spacing: 0.05em;
+  vertical-align: middle;
+}
+
 .ask-events {
   margin-bottom: 14px;
 }