feat: brain file upload button — PDFs, images, text files

- Upload button (arrow icon) in capture bar next to text input
- Accepts: PDF, PNG, JPG, GIF, WEBP, TXT, MD, CSV
- Multiple file upload supported
- Hidden file input triggered by button click
- Upload status indicator while processing
- Files sent to /api/brain/items/upload endpoint

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

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

@@ -37,6 +37,8 @@
   // Capture
   let captureInput = $state('');
   let capturing = $state(false);
+  let uploading = $state(false);
+  let fileInput: HTMLInputElement;
 
   // Detail
   let selectedItem = $state<BrainItem | null>(null);
@@ -119,6 +121,26 @@
     searching = false;
   }
 
+  async function uploadFile(e: Event) {
+    const input = e.target as HTMLInputElement;
+    if (!input.files?.length) return;
+    uploading = true;
+    try {
+      for (const file of input.files) {
+        const formData = new FormData();
+        formData.append('file', file);
+        await fetch('/api/brain/items/upload', {
+          method: 'POST',
+          credentials: 'include',
+          body: formData,
+        });
+      }
+      input.value = '';
+      await loadItems();
+    } catch { /* silent */ }
+    uploading = false;
+  }
+
   function startEditNote() {
     if (!selectedItem) return;
     editNoteContent = selectedItem.raw_content || '';
@@ -254,12 +276,19 @@
           onkeydown={handleCaptureKey}
           disabled={capturing}
         />
+        <button class="upload-btn" onclick={() => fileInput?.click()} disabled={uploading} title="Upload file">
+          <svg width="18" height="18" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><path d="M21 15v4a2 2 0 01-2 2H5a2 2 0 01-2-2v-4"/><polyline points="17 8 12 3 7 8"/><line x1="12" y1="3" x2="12" y2="15"/></svg>
+        </button>
         {#if captureInput.trim()}
           <button class="capture-btn" onclick={capture} disabled={capturing}>
             {capturing ? 'Saving...' : 'Save'}
           </button>
         {/if}
       </div>
+      <input bind:this={fileInput} type="file" class="hidden-input" accept=".pdf,.png,.jpg,.jpeg,.gif,.webp,.txt,.md,.csv" onchange={uploadFile} multiple />
+      {#if uploading}
+        <div class="upload-status">Uploading...</div>
+      {/if}
     </section>
 
     <!-- ═══ Folder signal strip ═══ -->
@@ -535,6 +564,26 @@
     color: #1e1812; font-size: 1rem; font-family: var(--font); outline: none;
   }
   .capture-input::placeholder { color: #8b7b6a; }
+  .upload-btn {
+    flex-shrink: 0;
+    width: 36px; height: 36px;
+    border-radius: 10px;
+    border: 1px solid rgba(35,26,17,0.1);
+    background: rgba(255,255,255,0.6);
+    color: #7f7365;
+    display: flex; align-items: center; justify-content: center;
+    transition: all 160ms;
+  }
+  .upload-btn:hover { background: rgba(255,255,255,0.9); color: #1e1812; border-color: rgba(35,26,17,0.2); }
+  .upload-btn:active { transform: scale(0.95); }
+
+  .hidden-input { display: none; }
+
+  .upload-status {
+    font-size: 0.82rem; color: #8c7b69;
+    margin-top: 8px; padding-left: 28px;
+  }
+
   .capture-btn {
     padding: 8px 18px; border-radius: 999px;
     background: #1e1812; color: white; border: none;
@@ -640,6 +689,7 @@
     height: auto;
     display: block;
     object-fit: cover;
+    object-position: top;
     max-height: 240px;
   }
   .card-type-badge {