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/layout/ThemeToggle.tsx

@@ -0,0 +1,50 @@
+"use client";
+
+import { useEffect, useState } from "react";
+
+const STORAGE_KEY = "pulsenode-theme";
+
+export function ThemeToggle() {
+  const [theme, setTheme] = useState<"dark" | "light" | null>(null);
+
+  useEffect(() => {
+    // One-time read of browser-only state (DOM attribute set by the anti-FOUC
+    // inline script, or the OS color-scheme preference) to resolve the theme
+    // React couldn't know during SSR. Not a case of syncing to external state
+    // that changes over time, so a single setState here is the right call.
+    const attr = document.documentElement.dataset.theme;
+    if (attr === "light" || attr === "dark") {
+      // eslint-disable-next-line react-hooks/set-state-in-effect
+      setTheme(attr);
+      return;
+    }
+    const prefersLight = window.matchMedia("(prefers-color-scheme: light)").matches;
+    setTheme(prefersLight ? "light" : "dark");
+  }, []);
+
+  function toggle() {
+    const next = theme === "light" ? "dark" : "light";
+    setTheme(next);
+    document.documentElement.dataset.theme = next;
+    try {
+      localStorage.setItem(STORAGE_KEY, next);
+    } catch {
+      // private browsing or storage disabled; toggle still works for this session
+    }
+  }
+
+  if (theme === null) {
+    return <div className="h-7 w-16 rounded-[var(--radius-widget)] border border-border bg-surface" />;
+  }
+
+  return (
+    <button
+      type="button"
+      onClick={toggle}
+      className="rounded-[var(--radius-widget)] border border-border bg-surface px-3 py-1.5 text-xs text-fg-muted transition-colors hover:text-fg"
+      aria-label="Toggle color theme"
+    >
+      {theme === "light" ? "Light" : "Dark"}
+    </button>
+  );
+}