Add tab bar with open/close/switch for multi-note editing

- Replace single selectedNote with tabs state in App (open/close/switch)
- Tab bar above editor: title + x close button, active tab highlighted
- Clicking note opens new tab or switches to existing
- Closing active tab switches to adjacent tab
- Fix race condition: editor re-creates when tab content loads async
- Add Playwright test verifying multi-tab behavior

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

src/components/Editor.css

@@ -21,6 +21,66 @@
   margin: 8px 0;
 }
 
+/* Tab bar */
+.editor__tab-bar {
+  display: flex;
+  background: #16162a;
+  border-bottom: 1px solid #2a2a4a;
+  overflow-x: auto;
+  flex-shrink: 0;
+}
+
+.editor__tab-bar::-webkit-scrollbar {
+  height: 0;
+}
+
+.editor__tab {
+  display: flex;
+  align-items: center;
+  gap: 6px;
+  padding: 6px 12px;
+  font-size: 12px;
+  color: #888;
+  cursor: pointer;
+  border-right: 1px solid #2a2a4a;
+  white-space: nowrap;
+  max-width: 180px;
+  flex-shrink: 0;
+}
+
+.editor__tab:hover {
+  background: #1e1e3a;
+  color: #ccc;
+}
+
+.editor__tab--active {
+  background: #0f0f1a;
+  color: #e0e0e0;
+  border-bottom: 2px solid #4a9eff;
+}
+
+.editor__tab-title {
+  overflow: hidden;
+  text-overflow: ellipsis;
+}
+
+.editor__tab-close {
+  background: none;
+  border: none;
+  color: #666;
+  cursor: pointer;
+  font-size: 14px;
+  padding: 0 2px;
+  line-height: 1;
+  border-radius: 3px;
+}
+
+.editor__tab-close:hover {
+  background: #3a3a5a;
+  color: #e0e0e0;
+}
+
+/* CodeMirror container */
 .editor__cm-container {
   flex: 1;
   overflow: auto;