Add Kamon logo as favicon and brand mark; fix MTU and UI issues

- Kamon logo (crest ring around the ghost) as favicon.svg, 180 px
  apple-touch-icon.png and the brand lockup in sidebar and login page,
  with the katakana reading ゴーストワイヤー
- /favicon.ico redirects to /favicon.svg
- Client configs no longer set an MTU; each device picks its own, as
  pivpn does. Server MTU changes no longer require reissued configs.
- Fix "null" rendered on the dashboard, wrapping activity times, and show
  log lines as readable text instead of raw JSON

Claude-Session: https://claude.ai/code/session_01RAnLbyQZ5ZTA7KqwXP98nw

index.html

@@ -5,6 +5,8 @@
 <meta name="viewport" content="width=device-width, initial-scale=1">
 <meta name="referrer" content="no-referrer">
 <title>GHOSTWIRE</title>
+<link rel="icon" href="/favicon.svg" type="image/svg+xml">
+<link rel="apple-touch-icon" href="/apple-touch-icon.png">
 <link rel="stylesheet" href="/app.css">
 <script src="/app.js" defer></script>
 </head>