fix(theme): dark theme + landing toggle now actually applies

Two bugs:
1. [data-theme="dark"] block was defined BEFORE :root in source CSS,
   so :root cascade overrode it. Moved to AFTER :root and bumped
   specificity to html[data-theme="dark"] to beat plain :root.
2. Theme toggle button only existed in the topbar, which is hidden
   on landing/login/sso-callback scenes. Added a ThemeToggle to
   the Landing header so users can flip theme from the home page.

Confidence: high
Scope-risk: narrow

src/scenes/Landing.tsx

@@ -2,7 +2,7 @@
 import { motion } from "framer-motion";
 import { useApp } from "../state/store";
 import { liveMeta } from "../data/scenarios";
-import { Sparkles, Arrow, Cmd, Bot, Pulse } from "../components/icons";
+import { Sparkles, Arrow, Cmd, Bot, Pulse, Sun, Moon } from "../components/icons";
 
 export default function Landing() {
   const setScene = useApp((s) => s.setScene);
@@ -28,9 +28,12 @@ export default function Landing() {
           <span className="brand-divider" />
           <span className="brand-sub">Mission Control</span>
         </div>
-        <button className="link-btn" onClick={() => setCmdOpen(true)}>
-          <Cmd size={13} /> Command <kbd>⌘K</kbd>
-        </button>
+        <div className="landing-top-actions">
+          <ThemeToggle />
+          <button className="link-btn" onClick={() => setCmdOpen(true)}>
+            <Cmd size={13} /> Command <kbd>⌘K</kbd>
+          </button>
+        </div>
       </header>
 
       <main className="landing-main">
@@ -132,3 +135,20 @@ export default function Landing() {
     </div>
   );
 }
+
+function ThemeToggle() {
+  const theme = useApp((s) => s.theme);
+  const setTheme = useApp((s) => s.setTheme);
+  const isDark = theme === "dark";
+  return (
+    <button
+      className="link-btn theme-toggle"
+      onClick={() => setTheme(isDark ? "light" : "dark")}
+      title={isDark ? "Switch to light theme" : "Switch to dark theme"}
+      aria-label="Toggle theme"
+    >
+      {isDark ? <Sun size={13} /> : <Moon size={13} />}
+      <span className="theme-toggle-label">{isDark ? "LIGHT" : "DARK"}</span>
+    </button>
+  );
+}