Show the offline screen when the network link is gone; plain panel

With no address on any interface (cable out, no wifi) the kiosk now
switches to the offline screen right away and returns when the network
and server are back. An unreachable server with the network up still
leaves the page running for InfoFlow's offline cache. The panel on the
local pages loses its blue top edge.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

www/kiosk.css

@@ -1,5 +1,5 @@
 /* Local pages shown on the kiosk screen itself (file://). Same look as the InfoFlow
-   display (dark theme): flat dark grey, white type, panels with a blue top edge. */
+   display (dark theme): flat dark grey, white type, plain panels. */
 :root {
   color-scheme: dark;
   --bg: #111827;
@@ -21,7 +21,7 @@ body { font-family: ui-sans-serif, system-ui, "DejaVu Sans", "Liberation Sans",
 .clock { font-size: clamp(36px, 8vmin, 110px); font-weight: 300; line-height: 1.1; font-variant-numeric: tabular-nums; }
 
 .middle { flex: 1; display: flex; align-items: center; justify-content: center; padding: 3vmin 0; }
-.panel { width: min(100%, 1100px); background: var(--panel); border-top: 4px solid var(--accent); border-radius: 8px; padding: 4.5vmin 5vmin; }
+.panel { width: min(100%, 1100px); background: var(--panel); border-radius: 8px; padding: 4.5vmin 5vmin; }
 h1 { font-size: clamp(26px, 4.6vmin, 64px); font-weight: 600; margin: 0 0 1.5vmin; }
 p { font-size: clamp(16px, 2.6vmin, 34px); color: var(--muted); margin: 0 0 1vmin; }
 .link { margin-top: 3.5vmin; padding-top: 2.5vmin; border-top: 1px solid var(--line); font-size: clamp(14px, 2.2vmin, 28px); color: var(--muted); overflow-wrap: anywhere; }