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

qa/probe_theme.mjs

@@ -0,0 +1,29 @@
+import { chromium } from "playwright";
+const b = await chromium.launch({
+  headless: true,
+  args: ["--host-resolver-rules=MAP canvas.flow-master.ai 65.21.71.186", "--ignore-certificate-errors"],
+});
+const ctx = await b.newContext({ viewport: { width: 1440, height: 900 } });
+const p = await ctx.newPage();
+await p.goto("https://canvas.flow-master.ai/", { waitUntil: "networkidle" });
+await p.waitForTimeout(2000);
+
+// Sign in first
+await p.locator("input[type='email']").fill("dev@flow-master.ai");
+const devBtn = p.locator("button", { hasText: /developer|dev-login/i });
+if (await devBtn.count() > 0) await devBtn.click();
+await p.waitForTimeout(2500);
+
+console.log("after login theme:", await p.evaluate(() => document.documentElement.dataset.theme));
+console.log("after login body bg:", await p.evaluate(() => getComputedStyle(document.body).backgroundColor));
+
+const toggle = p.locator(".theme-toggle, .link-btn.theme-toggle, button[aria-label*='theme']").first();
+const count = await toggle.count();
+console.log("toggle count:", count);
+if (count > 0) {
+  await toggle.click();
+  await p.waitForTimeout(800);
+  console.log("after click theme:", await p.evaluate(() => document.documentElement.dataset.theme));
+  console.log("after click body bg:", await p.evaluate(() => getComputedStyle(document.body).backgroundColor));
+}
+await b.close();