feat: brain tag filtering — click any tag to filter, clear pill, auto-poll

- Tags are now clickable buttons on cards and detail sheet
- Active tag filter shows as a pill with clear button
- Items filtered by tag via API query param
- Auto-polling every 3s for pending/processing items
- Detail sheet shows raw_content for notes with "Note" label

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

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

@@ -31,6 +31,9 @@
   let folders = $state<string[]>([]);
   let tags = $state<string[]>([]);
 
+  // Filter
+  let activeTag = $state<string | null>(null);
+
   // Capture
   let captureInput = $state('');
   let capturing = $state(false);
@@ -60,6 +63,7 @@
     try {
       const params = new URLSearchParams({ limit: '50' });
       if (activeFolder) params.set('folder', activeFolder);
+      if (activeTag) params.set('tag', activeTag);
       const data = await api(`/items?${params}`);
       items = data.items || [];
       total = data.total || 0;
@@ -256,6 +260,18 @@
       {/each}
     </section>
 
+    <!-- ═══ Active tag filter ═══ -->
+    {#if activeTag}
+      <div class="active-filter">
+        <span class="filter-label">Filtered by tag:</span>
+        <span class="filter-tag">{activeTag}</span>
+        <button class="filter-clear" onclick={() => { activeTag = null; 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>
+      </div>
+    {/if}
+
     <!-- ═══ Search bar ═══ -->
     <section class="search-section">
       <div class="search-wrap">
@@ -330,7 +346,7 @@
                 {#if item.tags && item.tags.length > 0}
                   <div class="card-tags">
                     {#each item.tags.slice(0, 3) as tag}
-                      <span class="card-tag">{tag}</span>
+                      <button class="card-tag" onclick={(e) => { e.stopPropagation(); activeTag = tag; activeFolder = null; loadItems(); }}>{tag}</button>
                     {/each}
                   </div>
                 {/if}
@@ -403,7 +419,7 @@
       {#if selectedItem.tags && selectedItem.tags.length > 0}
         <div class="detail-tags">
           {#each selectedItem.tags as tag}
-            <span class="detail-tag">{tag}</span>
+            <button class="detail-tag" onclick={() => { selectedItem = null; activeTag = tag; activeFolder = null; loadItems(); }}>{tag}</button>
           {/each}
         </div>
       {/if}
@@ -487,6 +503,27 @@
   .signal-value { font-size: clamp(1.6rem, 3vw, 2.2rem); line-height: 0.95; letter-spacing: -0.05em; color: #1e1812; }
   .signal-note { color: #4f463d; line-height: 1.6; font-size: 0.85rem; }
 
+  /* ═══ Active filter ═══ */
+  .active-filter {
+    display: flex;
+    align-items: center;
+    gap: 8px;
+    padding: 10px 16px;
+    margin-bottom: 12px;
+    border-radius: 999px;
+    background: rgba(179,92,50,0.08);
+    border: 1px solid rgba(179,92,50,0.15);
+    width: fit-content;
+  }
+  .filter-label { font-size: 0.82rem; color: #7d6f61; }
+  .filter-tag { font-size: 0.82rem; font-weight: 700; color: #1e1812; }
+  .filter-clear {
+    display: flex; align-items: center; gap: 3px;
+    font-size: 0.78rem; color: #8c7b69; background: none; border: none;
+    font-family: var(--font); transition: color 160ms;
+  }
+  .filter-clear:hover { color: #1e1812; }
+
   /* ═══ Search ═══ */
   .search-section { margin-bottom: 18px; }
   .search-wrap { position: relative; }
@@ -619,6 +656,14 @@
     font-size: 0.72rem;
     color: #5d5248;
     font-weight: 500;
+    border: none;
+    font-family: var(--font);
+    cursor: pointer;
+    transition: background 160ms, color 160ms;
+  }
+  .card-tag:hover {
+    background: rgba(179,92,50,0.12);
+    color: #1e1812;
   }
 
   .card-meta {
@@ -715,7 +760,10 @@
   .detail-tag {
     background: rgba(35,26,17,0.06); padding: 4px 12px;
     border-radius: 999px; font-size: 0.85rem; color: #5d5248;
+    border: none; font-family: var(--font); cursor: pointer;
+    transition: background 160ms;
   }
+  .detail-tag:hover { background: rgba(179,92,50,0.12); color: #1e1812; }
 
   .detail-actions { display: flex; gap: 8px; flex-wrap: wrap; }
   .action-btn {