feat(tenants): add workspace delete UI and fix project switch element loading (#11)

- Add DELETE /api/tenants/:id endpoint with cascade (projects, elements, snapshots)
- Add delete buttons with inline confirm in workspace switcher panel
- Fix project switch not loading elements: switchProjectUI now directly clears
  canvas and calls loadExistingElements() instead of relying on WS roundtrip

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/index.html

@@ -502,6 +502,12 @@
         }
         .menu-create-btn:hover:not(:disabled) { background: #4c2fa8; }
         .menu-create-btn:disabled { opacity: 0.5; cursor: default; }
+        .tenant-row {
+            position: relative;
+            display: flex;
+            align-items: stretch;
+        }
+        .tenant-row:hover .project-delete-btn { opacity: 0.5; }
         .project-row {
             position: relative;
             display: flex;