Add frameless window with custom drag regions (Obsidian-style)

Remove native macOS titlebar via decorations:false + titleBarStyle:overlay.
Add -webkit-app-region:drag to panel headers (Sidebar, NoteList, Editor tab
bar, Inspector) with no-drag on interactive elements. Sidebar header gets
78px left padding for macOS traffic light buttons.

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

src/components/Sidebar.css

@@ -8,9 +8,12 @@
 }
 
 .sidebar__header {
-  padding: 20px 16px 16px;
+  padding: 12px 16px;
+  padding-left: 78px; /* space for macOS traffic light buttons */
   border-bottom: 1px solid #2a2a4a;
   flex-shrink: 0;
+  -webkit-app-region: drag;
+  app-region: drag;
 }
 
 .sidebar__header h2 {
@@ -21,6 +24,11 @@
   letter-spacing: -0.01em;
 }
 
+.sidebar__header button {
+  -webkit-app-region: no-drag;
+  app-region: no-drag;
+}
+
 .sidebar__nav {
   padding: 8px 0;
   flex: 1;