fix: address security vulnerabilities from audit

- Path traversal: confine avatar file serving to avatar_dir (defaults to
  config file directory); validate on both read and write
- UDP owner injection: server-configured owner now takes precedence over
  UDP-supplied value, matching the documented intent
- Open redirect: reject non-relative next= values after login
- Stored XSS: enable Jinja2 autoescape on all template environments;
  add escHtml() helper in live.html and apply to all innerHTML sinks
  sourced from network data (host names, addrs, states, log messages)

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

hbd/server/udp.py

@@ -424,7 +424,7 @@ def handle_datagram(msg: dict, addr, transport, ctx: dict):
             if plugin_name == "os_info":
                 config_owner =  config_mod.get_host_access(cfg, uname).get("owner")
                 default_owner = config_mod.get_default_owner(cfg)
-                inferred_owner = plugin_data.get("owner", config_owner or default_owner)    
+                inferred_owner = config_owner or plugin_data.get("owner") or default_owner
                 host.owner = inferred_owner
                 logger.info(f"owner for {uname} is {host.owner}")
             if DEBUG > 1: