Fix content negotiation for browser navigation

Check Sec-Fetch-Mode: navigate header for direct browser access.
This ensures /cache/{hash} shows HTML detail page when navigating
directly, but still serves raw files for embedded <img>/<video> tags.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

server.py

@@ -3375,7 +3375,14 @@ async def get_user_from_cookie(request) -> Optional[str]:
 def wants_html(request: Request) -> bool:
     """Check if request wants HTML (browser) vs JSON (API)."""
     accept = request.headers.get("accept", "")
-    return "text/html" in accept and "application/json" not in accept
+    # Check for explicit HTML request
+    if "text/html" in accept and "application/json" not in accept:
+        return True
+    # Check for browser navigation (direct URL access)
+    fetch_mode = request.headers.get("sec-fetch-mode", "")
+    if fetch_mode == "navigate":
+        return True
+    return False
 
 
 # Tailwind CSS config for all L1 templates