feat: brain sidebar manage mode — add/delete folders and tags

- "Manage folders & tags" button at bottom of sidebar
- Click to enter manage mode:
  - + button appears next to Folders/Tags headers
  - Inline input to type and add new folder/tag
  - × delete button appears on each item
  - Delete confirms before removing
- "Done" button exits manage mode
- Tags/folders created/deleted via taxonomy API
- Sidebar refreshes after changes

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

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

@@ -341,26 +341,63 @@
         </nav>
 
         <div class="sidebar-separator"></div>
-        <div class="sidebar-section-label">Folders</div>
+        <div class="sidebar-section-head">
+          <div class="sidebar-section-label">Folders</div>
+          {#if showManage}
+            <button class="sidebar-section-add" onclick={() => { newTaxName = ''; sidebarView = 'folders'; }}>+</button>
+          {/if}
+        </div>
+        {#if showManage && sidebarView === 'folders'}
+          <div class="sidebar-add-row">
+            <input class="sidebar-add-input" placeholder="New folder..." bind:value={newTaxName} onkeydown={(e) => { if (e.key === 'Enter') addTaxonomy(); }} />
+            <button class="sidebar-add-go" onclick={addTaxonomy}>Add</button>
+          </div>
+        {/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(); }}>
               <span class="nav-label">{folder.name}</span>
-              {#if folder.item_count > 0}<span class="nav-count">{folder.item_count}</span>{/if}
+              {#if showManage}
+                <button class="nav-delete" onclick={(e) => { e.stopPropagation(); if (confirm(`Delete "${folder.name}"? Items will be moved.`)) deleteTaxonomy(folder.id); }}>×</button>
+              {:else if folder.item_count > 0}
+                <span class="nav-count">{folder.item_count}</span>
+              {/if}
             </button>
           {/each}
         </nav>
 
         <div class="sidebar-separator"></div>
-        <div class="sidebar-section-label">Tags</div>
+        <div class="sidebar-section-head">
+          <div class="sidebar-section-label">Tags</div>
+          {#if showManage}
+            <button class="sidebar-section-add" onclick={() => { newTaxName = ''; sidebarView = 'tags'; }}>+</button>
+          {/if}
+        </div>
+        {#if showManage && sidebarView === 'tags'}
+          <div class="sidebar-add-row">
+            <input class="sidebar-add-input" placeholder="New tag..." bind:value={newTaxName} onkeydown={(e) => { if (e.key === 'Enter') addTaxonomy(); }} />
+            <button class="sidebar-add-go" onclick={addTaxonomy}>Add</button>
+          </div>
+        {/if}
         <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(); }}>
               <span class="nav-label">{tag.name}</span>
-              <span class="nav-count">{tag.item_count}</span>
+              {#if showManage}
+                <button class="nav-delete" onclick={(e) => { e.stopPropagation(); if (confirm(`Delete tag "${tag.name}"?`)) deleteTaxonomy(tag.id); }}>×</button>
+              {:else if tag.item_count > 0}
+                <span class="nav-count">{tag.item_count}</span>
+              {/if}
             </button>
           {/each}
         </nav>
+
+        <div class="sidebar-separator"></div>
+        <div class="sidebar-manage-row">
+          <button class="sidebar-manage-btn" onclick={() => showManage = !showManage}>
+            {showManage ? 'Done' : 'Manage folders & tags'}
+          </button>
+        </div>
       </aside>
 
       <!-- Main content -->
@@ -761,10 +798,56 @@
   .nav-item.active .nav-count { color: #7f5f3d; opacity: 0.8; }
 
   .sidebar-separator { height: 1px; background: rgba(35,26,17,0.08); margin: 12px 18px; }
+
+  .sidebar-section-head {
+    display: flex; align-items: center; justify-content: space-between;
+    padding: 4px 18px 6px;
+  }
   .sidebar-section-label {
     font-size: 0.68rem; font-weight: 700; text-transform: uppercase;
-    letter-spacing: 0.14em; color: #8c7b69; padding: 4px 18px 6px;
+    letter-spacing: 0.14em; color: #8c7b69;
   }
+  .sidebar-section-add {
+    width: 20px; height: 20px; border-radius: 6px;
+    border: none; background: rgba(35,26,17,0.06);
+    color: #7f5f3d; font-size: 0.85rem; font-weight: 700;
+    display: flex; align-items: center; justify-content: center;
+    cursor: pointer; transition: all 140ms;
+  }
+  .sidebar-section-add:hover { background: rgba(35,26,17,0.12); }
+
+  .sidebar-add-row {
+    display: flex; gap: 4px; padding: 0 12px; margin-bottom: 6px;
+  }
+  .sidebar-add-input {
+    flex: 1; padding: 6px 10px; border-radius: 10px;
+    border: 1px solid rgba(35,26,17,0.12); background: rgba(255,255,255,0.7);
+    font-size: 0.8rem; font-family: var(--font); color: #1e1812; outline: none;
+  }
+  .sidebar-add-input:focus { border-color: rgba(179,92,50,0.4); }
+  .sidebar-add-input::placeholder { color: #8c7b69; }
+  .sidebar-add-go {
+    padding: 6px 10px; border-radius: 10px; border: none;
+    background: #1e1812; color: white; font-size: 0.75rem; font-weight: 600;
+    font-family: var(--font); cursor: pointer;
+  }
+
+  .nav-delete {
+    width: 22px; height: 22px; border-radius: 6px;
+    border: none; background: none; color: #8c7b69;
+    font-size: 0.9rem; display: flex; align-items: center; justify-content: center;
+    cursor: pointer; transition: all 140ms; flex-shrink: 0;
+  }
+  .nav-delete:hover { background: rgba(220,38,38,0.1); color: #DC2626; }
+
+  .sidebar-manage-row { padding: 0 12px; }
+  .sidebar-manage-btn {
+    width: 100%; padding: 8px; border-radius: 10px;
+    border: 1px solid rgba(35,26,17,0.08); background: none;
+    font-size: 0.78rem; color: #8c7b69; font-family: var(--font);
+    cursor: pointer; transition: all 140ms;
+  }
+  .sidebar-manage-btn:hover { background: rgba(255,248,241,0.72); color: #1f1811; }
 
   .brain-content {
     flex: 1;