feat: brain mobile sidebar — slide-out drawer with overlay

- "Folders & Tags" button shown only on mobile
- Click opens sidebar as slide-out drawer from left
- Dark overlay behind, click to dismiss
- Selecting a folder/tag auto-closes the drawer
- Desktop sidebar unchanged

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend-v2/src/lib/pages/brain/AtelierBrainPage.svelte

@@ -40,6 +40,7 @@
   let sidebarFolders = $state<SidebarFolder[]>([]);
   let sidebarTags = $state<SidebarTag[]>([]);
   let sidebarView = $state<'folders' | 'tags'>('folders');
+  let mobileSidebarOpen = $state(false);
   let showManage = $state(false);
   let newTaxName = $state('');
 
@@ -327,7 +328,11 @@
 <div class="brain-layout">
 
       <!-- Second sidebar (like Reader) -->
-      <aside class="brain-sidebar">
+      <!-- svelte-ignore a11y_no_static_element_interactions -->
+      {#if mobileSidebarOpen}
+        <div class="mobile-sidebar-overlay" onclick={() => mobileSidebarOpen = false}></div>
+      {/if}
+      <aside class="brain-sidebar" class:mobile-open={mobileSidebarOpen}>
         <div class="sidebar-header">
           <svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round"><path d="M12 2L2 7l10 5 10-5-10-5z"/><path d="M2 17l10 5 10-5"/><path d="M2 12l10 5 10-5"/></svg>
           <span class="sidebar-title">Brain</span>
@@ -341,7 +346,7 @@
         </div>
 
         <nav class="sidebar-nav">
-          <button class="nav-item" class:active={!activeFolder && !activeTag} onclick={() => { activeFolder = null; activeTag = null; activeFolderId = null; activeTagId = null; loadItems(); }}>
+          <button class="nav-item" class:active={!activeFolder && !activeTag} onclick={() => { activeFolder = null; activeTag = null; activeFolderId = null; activeTagId = null; mobileSidebarOpen = false; loadItems(); }}>
             <span class="nav-label">All items</span>
             <span class="nav-count">{total}</span>
           </button>
@@ -360,7 +365,7 @@
         {/if}
         <nav class="sidebar-nav">
           {#each sidebarFolders.filter(f => f.is_active) as folder}
-            <button class="nav-item" class:active={activeFolder === folder.name} onclick={() => { activeFolder = folder.name; activeFolderId = folder.id; activeTag = null; activeTagId = null; loadItems(); }}>
+            <button class="nav-item" class:active={activeFolder === folder.name} onclick={() => { activeFolder = folder.name; activeFolderId = folder.id; activeTag = null; activeTagId = null; mobileSidebarOpen = false; loadItems(); }}>
               <span class="nav-label">{folder.name}</span>
               {#if showManage}
                 <!-- svelte-ignore a11y_no_static_element_interactions --><span class="nav-delete" onclick={(e) => { e.stopPropagation(); if (confirm(`Delete "${folder.name}"? Items will be moved.`)) deleteTaxonomy(folder.id); }}>×</span>
@@ -383,7 +388,7 @@
           </div>
           <nav class="sidebar-nav">
             {#each sidebarTags.filter(t => t.is_active) as tag}
-              <button class="nav-item" class:active={activeTag === tag.name} onclick={() => { activeTag = tag.name; activeTagId = tag.id; activeFolder = null; activeFolderId = null; loadItems(); }}>
+              <button class="nav-item" class:active={activeTag === tag.name} onclick={() => { activeTag = tag.name; activeTagId = tag.id; activeFolder = null; activeFolderId = null; mobileSidebarOpen = false; loadItems(); }}>
                 <span class="nav-label">{tag.name}</span>
                 <!-- svelte-ignore a11y_no_static_element_interactions --><span class="nav-delete" onclick={(e) => { e.stopPropagation(); if (confirm(`Delete tag "${tag.name}"?`)) deleteTaxonomy(tag.id); }}>×</span>
               </button>
@@ -392,7 +397,7 @@
         {:else}
           <nav class="sidebar-nav">
             {#each sidebarTags.filter(t => t.is_active && t.item_count > 0) as tag}
-              <button class="nav-item" class:active={activeTag === tag.name} onclick={() => { activeTag = tag.name; activeTagId = tag.id; activeFolder = null; activeFolderId = null; loadItems(); }}>
+              <button class="nav-item" class:active={activeTag === tag.name} onclick={() => { activeTag = tag.name; activeTagId = tag.id; activeFolder = null; activeFolderId = null; mobileSidebarOpen = false; loadItems(); }}>
                 <span class="nav-label">{tag.name}</span>
                 <span class="nav-count">{tag.item_count}</span>
               </button>
@@ -432,12 +437,18 @@
           {#if uploading}<div class="upload-status">Uploading...</div>{/if}
         </section>
 
+        <!-- Mobile sidebar toggle -->
+        <button class="mobile-filter-btn" onclick={() => mobileSidebarOpen = true}>
+          <svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round"><line x1="4" y1="21" x2="4" y2="14"/><line x1="4" y1="10" x2="4" y2="3"/><line x1="12" y1="21" x2="12" y2="12"/><line x1="12" y1="8" x2="12" y2="3"/><line x1="20" y1="21" x2="20" y2="16"/><line x1="20" y1="12" x2="20" y2="3"/></svg>
+          Folders & Tags
+        </button>
+
         <!-- Active filter indicator -->
         {#if activeFolder || activeTag}
         <div class="active-filter">
           <span class="filter-label">Filtered by {activeFolder ? 'folder' : 'tag'}:</span>
           <span class="filter-tag">{activeFolder || activeTag}</span>
-          <button class="filter-clear" onclick={() => { activeFolder = null; activeTag = null; activeFolderId = null; activeTagId = null; loadItems(); }}>
+          <button class="filter-clear" onclick={() => { activeFolder = null; activeTag = null; activeFolderId = null; activeTagId = null; mobileSidebarOpen = false; loadItems(); }}>
             <svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><line x1="18" y1="6" x2="6" y2="18"/><line x1="6" y1="6" x2="18" y2="18"/></svg>
             Clear
           </button>
@@ -856,6 +867,28 @@
     overflow-x: hidden;
   }
 
+  /* ═══ Mobile filter button ═══ */
+  .mobile-filter-btn {
+    display: none;
+    align-items: center;
+    gap: 6px;
+    padding: 8px 14px;
+    border-radius: 10px;
+    border: 1px solid rgba(35,26,17,0.1);
+    background: rgba(255,252,248,0.7);
+    color: #5c5046;
+    font-size: 0.82rem;
+    font-family: var(--font);
+    cursor: pointer;
+    margin-bottom: 12px;
+    transition: all 160ms;
+  }
+  .mobile-filter-btn:hover { background: rgba(255,248,241,0.9); color: #1e1812; }
+
+  @media (max-width: 768px) {
+    .mobile-filter-btn { display: flex; }
+  }
+
   /* ═══ Active filter ═══ */
   .active-filter {
     display: flex;
@@ -1288,7 +1321,16 @@
   @media (max-width: 768px) {
     .brain-command { display: grid; gap: 14px; }
     .command-actions { justify-items: start; }
-    .brain-sidebar { display: none; }
+    .brain-sidebar {
+      display: none;
+      position: fixed; left: 0; top: 0; bottom: 0; z-index: 50;
+      width: 280px; box-shadow: 8px 0 24px rgba(0,0,0,0.1);
+    }
+    .brain-sidebar.mobile-open { display: flex; }
+    .mobile-sidebar-overlay {
+      position: fixed; inset: 0; z-index: 49;
+      background: rgba(17,13,10,0.3);
+    }
     .brain-layout { margin: 0; }
     .masonry { columns: 1; }
     .detail-sheet { width: 100%; padding: 20px; }