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

web.go

@@ -5,19 +5,22 @@ import (
 	"net/http"
 )
 
-// The web UI is three static files built into the binary. It talks only to
+// The web UI and its icons are built into the binary. The UI talks only to
 // /api/v1, the same API the iOS app uses.
 //
-//go:embed index.html app.js app.css
+//go:embed index.html app.js app.css favicon.svg apple-touch-icon.png
 var webFiles embed.FS
 
 func webHandler() http.Handler {
 	files := http.FileServerFS(webFiles)
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		switch r.URL.Path {
-		case "/", "/app.js", "/app.css":
+		case "/", "/app.js", "/app.css", "/favicon.svg", "/apple-touch-icon.png":
 			w.Header().Set("Cache-Control", "no-cache")
 			files.ServeHTTP(w, r)
+		case "/favicon.ico":
+			// Browsers ask for this by default; point them to the SVG.
+			http.Redirect(w, r, "/favicon.svg", http.StatusMovedPermanently)
 		default:
 			http.NotFound(w, r)
 		}