feat: add sidebar collapse toggle for compact icon-rail mode (#335)

Add a collapse button at the top of the left sidebar that toggles
between full-width (240px with icons + labels) and compact icon-only
mode (64px). The collapse state is persisted to localStorage and
survives page refreshes.

Changes:
- Add collapse toggle button with chevron icon in sidebar
- Add CSS rules for .sidebar.collapsed: narrow width, centered icons,
  hidden labels/group-headers/footer-text
- Leverage existing store state (sidebarCollapsed, toggleSidebarCollapsed)
  and SCSS variable ($sidebar-collapsed-width) that were already defined
  but never wired up in the template

This significantly improves browsing experience on tablets and
handheld devices by freeing up horizontal space when the full
sidebar is not needed.

packages/client/src/components/layout/AppSidebar.vue

@@ -57,13 +57,20 @@ function openChangelog() {
 </script>
 
 <template>
-  <aside class="sidebar" :class="{ open: appStore.sidebarOpen }">
+  <aside class="sidebar" :class="{ open: appStore.sidebarOpen, collapsed: appStore.sidebarCollapsed }">
     <div class="sidebar-logo" @click="router.push('/hermes/chat')">
       <img :src="logoPath" alt="Hermes" class="logo-img" />
       <span class="logo-text">Hermes</span>
       <!-- <video class="logo-dance" :src="isDark ? danceVideoDark : danceVideoLight" autoplay loop muted playsinline /> -->
     </div>
 
+    <button class="collapse-btn" @click="appStore.toggleSidebarCollapsed()" :title="appStore.sidebarCollapsed ? t('sidebar.expand') : t('sidebar.collapse')">
+      <svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+        <polyline v-if="appStore.sidebarCollapsed" points="9 18 15 12 9 6" />
+        <polyline v-else points="15 18 9 12 15 6" />
+      </svg>
+    </button>
+
     <nav class="sidebar-nav">
       <!-- Conversation -->
       <div class="nav-group">
@@ -316,6 +323,7 @@ function openChangelog() {
 @use "@/styles/variables" as *;
 
 .sidebar {
+  position: relative;
   width: $sidebar-width;
   height: calc(100 * var(--vh));
   background-color: $bg-sidebar;
@@ -602,6 +610,112 @@ function openChangelog() {
   }
 }
 
+// ─── Collapsed sidebar (icon-rail mode) ─────────────────────────
+
+.sidebar.collapsed {
+  width: $sidebar-collapsed-width;
+  padding: 0 8px 12px;
+  overflow: hidden;
+
+  .sidebar-logo {
+    padding: 12px 4px 8px;
+    margin: 0 -8px;
+    justify-content: center;
+    gap: 0;
+
+    .logo-text {
+      display: none;
+    }
+  }
+
+  .collapse-btn {
+    display: flex;
+    margin: 0 auto 8px;
+  }
+
+  .nav-group-label {
+    display: none;
+  }
+
+  .nav-item {
+    justify-content: center;
+    padding: 10px 4px;
+    gap: 0;
+
+    span {
+      display: none;
+    }
+
+    svg {
+      flex-shrink: 0;
+    }
+  }
+
+  // Keep group children visible — user can still see icons
+  .nav-group > div {
+    display: flex !important;
+    flex-direction: column;
+    gap: 2px;
+  }
+
+  // Hide selectors and footer text, keep theme switch
+  :deep(.profile-selector),
+  :deep(.model-selector) {
+    display: none;
+  }
+
+  .sidebar-footer {
+    .logout-item span {
+      display: none;
+    }
+
+    .status-text {
+      display: none;
+    }
+
+    .version-text,
+    .github-link {
+      display: none;
+    }
+
+    .status-row {
+      justify-content: center;
+    }
+  }
+}
+
+// ─── Collapse button ────────────────────────────────────────────
+
+.collapse-btn {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  width: 28px;
+  height: 28px;
+  border: none;
+  background: none;
+  color: $text-muted;
+  border-radius: $radius-sm;
+  cursor: pointer;
+  flex-shrink: 0;
+  margin-left: auto;
+  margin-right: 0;
+  transition: all $transition-fast;
+
+  &:hover {
+    color: $text-primary;
+    background-color: rgba(var(--accent-primary-rgb), 0.08);
+  }
+}
+
+// In expanded mode, overlap the top-right of the logo area
+.sidebar:not(.collapsed) .collapse-btn {
+  position: absolute;
+  top: 18px;
+  right: 16px;
+  z-index: 5;
+}
+
 @media (max-width: $breakpoint-mobile) {
   .logo-dance {
     display: none;