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/index.html

@@ -180,6 +180,19 @@
             <button id="note-vis-btn" title="Toggle note visibility (private / public)">prv</button>
             <!-- Tools toggle — routes through the orchestrator tool loop when active -->
             <button id="tools-toggle" title="Tools disabled — click to enable">⚡</button>
+            <!-- Attach file — images (vision) or text/code files -->
+            <button id="attach-btn" title="Attach image or text file">📎</button>
+            <input type="file" id="file-input" style="display:none"
+                   accept="image/png,image/jpeg,image/webp,image/gif,text/plain,text/markdown,.md,.txt,.py,.js,.ts,.jsx,.tsx,.json,.yaml,.yml,.toml,.html,.css,.sh,.csv,.xml,.rs,.go,.java,.c,.cpp,.h,.rb,.php,.swift,.kt,.sql">
+        </div>
+        <!-- Attachment preview — shown when a file is pending -->
+        <div id="attachment-row" style="display:none">
+            <div id="attachment-preview">
+                <img id="attachment-thumb" alt="" style="display:none">
+                <span id="attachment-icon">📎</span>
+                <span id="attachment-name"></span>
+                <button id="attachment-clear" title="Remove attachment">✕</button>
+            </div>
         </div>
         <textarea id="input" rows="1" placeholder="Message…" autofocus></textarea>
         <div id="send-col">