feat: visual identity, theme toggle, custom CSS, loading states (M4)

Grounds the visual design in the product's own subject matter (vital-
signs monitoring) rather than generic dark-mode defaults: a signature
"pulse" green tied to healthy status (the accent color and the "up"
status color are the same hue - a steady green pulse reads as
healthy, same convention as a cardiac monitor), a small ECG-trace
brand mark, and a restrained pulsing-glow animation on healthy status
dots (prefers-reduced-motion respected) as the one deliberate motion
touch. Typography moves off the bare system-ui stack: IBM Plex Sans
for UI text, IBM Plex Mono with tabular numerals for metric values
(CPU/mem/uptime/latency), Space Grotesk used once for the wordmark -
chosen partly for IBM Plex's own systems-monitoring heritage.

Functional additions:
- Light/dark toggle, independent of config.yml's theme.mode, persisted
  to localStorage with a blocking inline script to avoid a flash of
  the wrong theme on load; auto mode now genuinely follows
  prefers-color-scheme instead of hardcoding dark.
- theme.customCssPath support via a new route handler that serves a
  user-mounted CSS file at runtime (can't be a build-time import,
  hence the targeted no-css-tags lint suppression), with a path-
  traversal guard since it's still reading from disk on every request.
- Loading-skeleton state for widgets awaiting their first WebSocket
  result, distinct from both the error state and genuine no-data.

Verified visually in Chrome: theme toggle switches instantly in both
directions, a live docker widget's status dot and monospace metrics
render correctly in both palettes, and config hot-reload still adds a
new widget without a page refresh.

components/widgets/http/Widget.tsx

@@ -2,6 +2,7 @@
 
 import type { Widget } from "@/lib/config/schema";
 import { useWidgetSubscription } from "@/lib/ws/client";
+import { SkeletonLines } from "@/components/widgets/Skeleton";
 
 type HttpWidget = Extract<Widget, { type: "http" }>;
 
@@ -15,21 +16,27 @@ export function HttpWidget({ widget, widgetId }: { widget: HttpWidget; widgetId:
   const result = useWidgetSubscription(widgetId);
   const data = result?.type === "http" ? result.data : null;
   const errorMessage = result?.type === "error" ? result.message : null;
+  const pulse = data?.up ? "status-pulse" : "";
 
   return (
     <div className="flex flex-col gap-2 rounded-[var(--radius-widget)] border border-border bg-surface p-4">
       <div className="flex items-center justify-between gap-2">
         <span className="text-sm font-medium text-fg">{widget.name}</span>
         <span
-          className={`h-2.5 w-2.5 shrink-0 rounded-full ${statusColor(data?.up, data?.consecutiveFailures ?? 0)}`}
+          className={`h-2.5 w-2.5 shrink-0 rounded-full ${statusColor(data?.up, data?.consecutiveFailures ?? 0)} ${pulse}`}
           title={data ? (data.up ? "up" : `down (${data.consecutiveFailures} checks failed)`) : "unknown"}
         />
       </div>
       {errorMessage && <span className="text-xs text-status-down">{errorMessage}</span>}
+      {!result && !errorMessage && <SkeletonLines />}
       {data && (
         <dl className="flex flex-col gap-1 text-xs text-fg-muted">
-          <div>Status: {data.statusCode ?? "—"}</div>
-          <div>Latency: {data.latencyMs}ms</div>
+          <div>
+            Status: <span className="font-mono tabular-nums text-fg">{data.statusCode ?? "—"}</span>
+          </div>
+          <div>
+            Latency: <span className="font-mono tabular-nums text-fg">{data.latencyMs}ms</span>
+          </div>
           {!data.up && data.error && <div className="text-status-down">{data.error}</div>}
         </dl>
       )}