feat(sidebar): Notion-style collapse/peek, desktop reopen button, Ctrl+\\ toggle

Stop collapsing the sidebar on navigation (was hiding the nav when opening Shared pages). Show the hamburger in the top-left on desktop when collapsed, add a left-edge hover zone that floats the sidebar in and auto-collapses on mouse leave, and bind Ctrl/Cmd+\\ to toggle it.

static/css/app.css

@@ -166,6 +166,31 @@ input, select, textarea { font-family: inherit; color: inherit; }
   padding: 0;
 }
 
+/* ── Notion-style collapsed peek (hover the left edge) ── */
+.sidebar-peek-zone {
+  position: fixed;
+  top: 0;
+  left: 0;
+  bottom: 0;
+  width: 8px;
+  z-index: 1000;
+}
+.sidebar.peeking {
+  position: fixed;
+  top: 0;
+  left: 0;
+  bottom: 0;
+  width: var(--sidebar-width);
+  min-width: var(--sidebar-width);
+  z-index: 1001;
+  box-shadow: 4px 0 24px rgba(0, 0, 0, .35);
+  animation: fd-sidebar-peek-in .16s ease-out;
+}
+@keyframes fd-sidebar-peek-in {
+  from { transform: translateX(-100%); }
+  to { transform: translateX(0); }
+}
+
 /* Workspace header — Notion style */
 .sidebar-workspace-header {
   display: flex;
@@ -3530,6 +3555,12 @@ button.breadcrumb-current { max-width: 260px; }
 }
 .hamburger-btn:hover { background: var(--bg-hover); }
 
+/* Show the hamburger on desktop only when the sidebar is collapsed,
+   so it can be reopened from the top-left corner. */
+@media (min-width: 769px) {
+  .hamburger-btn.hamburger-desktop-show { display: flex; }
+}
+
 /* ── Mobile Sidebar Overlay ── */
 .sidebar-overlay {
   display: none;
@@ -3611,6 +3642,12 @@ button.breadcrumb-current { max-width: 260px; }
   }
   .sidebar-back-btn { display: flex; }
   .sidebar-uncollapse-btn { display: none !important; }
+  .sidebar-peek-zone { display: none; }
+  .sidebar.peeking {
+    width: 85vw;
+    max-width: 320px;
+    min-width: 0;
+  }
 
   /* ── Topbar ── */
   .topbar {