feat: add vault download button and markdown viewer PRD to /files page

Adds GET /files/download endpoint that streams all vault files (excl.
.git) as a deflate-compressed vault.zip. Adds Download All button to
the /files page header. Also adds FILES_VIEWER_PRD.md planning doc.

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

webui.py

@@ -12,11 +12,13 @@ Module-level VAULT_DIR and SETTINGS_PATH can be overridden by tests.
 import asyncio
 import base64
 import difflib
+import io
 import json
 import os
 import re
 import subprocess
 import uuid
+import zipfile
 from pathlib import Path
 from typing import Optional
 
@@ -797,6 +799,23 @@ async def sync_history(request: Request):
 # File browser
 # ---------------------------------------------------------------------------
 
+@app.get("/files/download")
+async def download_vault():
+    if not VAULT_DIR.exists():
+        raise HTTPException(status_code=404, detail="Vault directory not found")
+    buf = io.BytesIO()
+    with zipfile.ZipFile(buf, "w", zipfile.ZIP_DEFLATED) as zf:
+        for file_path in sorted(VAULT_DIR.rglob("*")):
+            if file_path.is_file() and ".git" not in file_path.parts:
+                zf.write(file_path, file_path.relative_to(VAULT_DIR))
+    buf.seek(0)
+    return StreamingResponse(
+        buf,
+        media_type="application/zip",
+        headers={"Content-Disposition": 'attachment; filename="vault.zip"'},
+    )
+
+
 @app.get("/files", response_class=HTMLResponse)
 async def file_browser():
     tree_html = _get_vault_tree_html()
@@ -811,7 +830,10 @@ async def file_browser():
 <div class="card">
   <div style="display:flex;justify-content:space-between;align-items:center;margin-bottom:12px">
     <h2 style="margin:0">Vault Files ({file_count})</h2>
-    <small style="color:#999"><code>{VAULT_DIR}</code></small>
+    <div style="display:flex;align-items:center;gap:12px">
+      <a href="/files/download" class="btn btn-secondary" style="font-size:.8rem;padding:6px 14px">&#x2B73; Download All</a>
+      <small style="color:#999"><code>{VAULT_DIR}</code></small>
+    </div>
   </div>
   <div style="font-size:.9rem;line-height:1.6">{tree_html}</div>
 </div>"""