Sidebar becomes a mobile drawer below 768px

The sidebar was flex:0 0 15rem inside .app__body unconditionally, so
on a narrow viewport it and the main content just fought over space.
Below 768px it now leaves the flex flow entirely (position: fixed,
translated off-canvas by default) and slides in as a drawer instead,
leaving .app__main the full width.

- App.vue: a ☰ button in the header (hidden above the breakpoint)
  opens it; a backdrop tap, the drawer's own close button, or any
  navigation (route.fullPath watcher) closes it. State lives in
  App.vue since the toggle button is in the header, not the sidebar.
- AppSidebar.vue: a close button (✕), only visible in the drawer,
  emitting 'close'.
- Desktop (>=768px) layout and behaviour is untouched -- the drawer
  CSS and the toggle button both live behind the same media query.

Drag-and-drop is untouched, as asked -- this only affects layout, and
the sidebar's inbox draggable works the same as before either way.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/src/style.css

@@ -62,10 +62,36 @@ body {
   border-bottom: 1px solid var(--border);
 }
 
+.app__bar-left {
+  display: flex;
+  align-items: center;
+  gap: 0.75rem;
+  min-width: 0;
+}
+
 .app__brand {
   font-weight: 600;
 }
 
+/* Opens the sidebar drawer on mobile (see the sidebar's own mobile rules
+   below) -- hidden here, shown only under that same breakpoint. */
+.app__menu-toggle {
+  display: none;
+  flex: none;
+  border: 1px solid var(--border);
+  background: var(--bg);
+  color: var(--text);
+  border-radius: 8px;
+  padding: 0.35rem 0.55rem;
+  font-size: 1rem;
+  line-height: 1;
+  cursor: pointer;
+}
+
+.app__menu-toggle:hover {
+  border-color: var(--muted);
+}
+
 .app__account {
   display: flex;
   align-items: center;
@@ -118,6 +144,70 @@ body {
   background: var(--surface);
 }
 
+/* Only shown once .sidebar becomes a drawer, under the mobile breakpoint
+   below -- hidden here. */
+.sidebar__close {
+  display: none;
+  position: absolute;
+  top: 0.75rem;
+  right: 0.75rem;
+  border: none;
+  background: none;
+  color: var(--muted);
+  cursor: pointer;
+  font-size: 1rem;
+  padding: 0.3rem 0.5rem;
+  border-radius: 6px;
+}
+
+.sidebar__close:hover {
+  background: var(--bg);
+  color: var(--text);
+}
+
+/* --- mobile: sidebar becomes an off-canvas drawer ---------------------- */
+
+@media (max-width: 768px) {
+  .app__menu-toggle {
+    display: inline-flex;
+  }
+
+  .app__body {
+    padding: 1rem;
+  }
+
+  .sidebar {
+    position: fixed;
+    top: 0;
+    left: 0;
+    z-index: 200;
+    width: 85vw;
+    max-width: 20rem;
+    height: 100vh;
+    border: none;
+    border-radius: 0;
+    border-right: 1px solid var(--border);
+    padding-top: 2.75rem;
+    transform: translateX(-100%);
+    transition: transform 0.2s ease;
+  }
+
+  .sidebar--open {
+    transform: translateX(0);
+  }
+
+  .sidebar__close {
+    display: block;
+  }
+
+  .app__drawer-backdrop {
+    position: fixed;
+    inset: 0;
+    z-index: 150;
+    background: rgba(0, 0, 0, 0.45);
+  }
+}
+
 .sidebar__nav {
   display: flex;
   flex-direction: column;