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/traefik/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 TraefikWidget = Extract<Widget, { type: "traefik" }>;
 
@@ -15,12 +16,13 @@ export function TraefikWidget({ widget, widgetId }: { widget: TraefikWidget; wid
       <div className="flex items-center justify-between">
         <span className="text-sm font-medium text-fg">{widget.name}</span>
         {data && (
-          <span className="text-xs text-fg-muted">
+          <span className="font-mono text-xs tabular-nums text-fg-muted">
             {data.entrypoints.map((ep) => ep.address).join(" · ") || `${data.entrypoints.length} entrypoints`}
           </span>
         )}
       </div>
       {errorMessage && <span className="text-xs text-status-down">{errorMessage}</span>}
+      {!result && !errorMessage && <SkeletonLines count={3} />}
       {data && (
         <div className="flex flex-col gap-1">
           {data.routers.map((router) => (
@@ -38,7 +40,9 @@ export function TraefikWidget({ widget, widgetId }: { widget: TraefikWidget; wid
               </span>
             </div>
           ))}
-          <span className="pt-1 text-xs text-fg-muted">{data.middlewaresCount} middlewares</span>
+          <span className="pt-1 text-xs text-fg-muted">
+            <span className="font-mono tabular-nums">{data.middlewaresCount}</span> middlewares
+          </span>
         </div>
       )}
     </div>