feat: file attachment support in chat (images + text/code files)

Text files (.md, .py, .js, .json, etc.): read client-side and injected
into the message body as a fenced code block — works with all backends
with zero model capability requirements.

Images (PNG/JPG/WebP/GIF, max 5 MB): encoded as base64 data URL on the
client and sent as a separate attachment field. Backend formats them as
OpenAI multimodal content (text + image_url) for local_openai backends.
Claude CLI and Gemini CLI see the text message with a "📎 filename.png"
note; image data is never written to session history.

- index.html: 📎 button + hidden file input in mode-select row;
  attachment-row preview area with thumbnail (images) or filename chip
- app.js: _resolveAttachment(), file reader, clearAttachment();
  sendMessage/sendOrchestrate updated to allow no-text sends when a
  file is pending; attachment spread into chat payload for images
- chat.py: Attachment model; attachment field on ChatRequest;
  llm_attachment extracted in _stream_chat and passed to complete()
- llm_client.py: attachment param through complete()/_dispatch()/_local();
  _local() builds multimodal content array for vision calls
- style.css: #attach-btn, #attachment-row, #attachment-preview, thumb

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cortex/static/style.css

@@ -861,6 +861,58 @@
         }
         #tools-toggle.local-on:hover { box-shadow: 0 0 10px var(--amber-glow); }
 
+        #attach-btn {
+            background: var(--bg);
+            border: 1px solid rgba(255,255,255,0.1);
+            border-radius: 6px;
+            color: rgba(255,255,255,0.3);
+            font-size: 0.95rem;
+            padding: 3px 7px;
+            cursor: pointer;
+            transition: color 0.15s, border-color 0.15s;
+        }
+        #attach-btn:hover { color: rgba(255,255,255,0.6); border-color: rgba(255,255,255,0.25); }
+
+        #attachment-row {
+            padding: 0.3rem 0.5rem;
+            border-bottom: 1px solid var(--border);
+        }
+        #attachment-preview {
+            display: inline-flex;
+            align-items: center;
+            gap: 0.4rem;
+            background: var(--bg-alt);
+            border: 1px solid var(--border);
+            border-radius: 6px;
+            padding: 0.2rem 0.5rem;
+            font-size: 0.82rem;
+            max-width: 100%;
+        }
+        #attachment-thumb {
+            max-height: 2.4rem;
+            max-width: 3.5rem;
+            border-radius: 3px;
+            object-fit: contain;
+        }
+        #attachment-name {
+            color: var(--text-mid);
+            max-width: 220px;
+            overflow: hidden;
+            text-overflow: ellipsis;
+            white-space: nowrap;
+        }
+        #attachment-clear {
+            background: none;
+            border: none;
+            color: var(--muted);
+            cursor: pointer;
+            padding: 0 0.15rem;
+            font-size: 0.78rem;
+            line-height: 1;
+            flex-shrink: 0;
+        }
+        #attachment-clear:hover { color: var(--text); }
+
         #input {
             flex: 1;
             background: var(--bg);