feat(web): marketing theme toggle, site search palette, and phone dev access

- Theme toggle in the nav: follows the device until chosen, remembers the choice, sets data-theme before first paint (no flash).
- Site search: Cmd/Ctrl+K command palette on every marketing page, opened by the shortcut, a nav Search button, or the Docs search field. Searches pages, docs topics and articles that have a page.
- Docs topics move to content.ts so the page and the search index share one source.
- next.config: allowedDevOrigins for private network ranges so a phone on the same Wi-Fi hydrates the dev site. Dev only.
- Sun and moon icons added by hand (not Figma exports); noted in the icons README.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

web/__tests__/marketing/theme.test.ts

@@ -0,0 +1,59 @@
+import { beforeEach, describe, expect, it } from 'vitest';
+import { THEME_KEY, applyTheme, currentTheme, readStoredTheme, themeInitScript } from '../../src/marketing/theme.ts';
+
+describe('theme', () => {
+  beforeEach(() => {
+    localStorage.clear();
+    document.documentElement.removeAttribute('data-theme');
+  });
+
+  it('has no stored theme until the visitor chooses one', () => {
+    expect(readStoredTheme()).toBeNull();
+  });
+
+  it('ignores a stored value that is not light or dark', () => {
+    localStorage.setItem(THEME_KEY, 'purple');
+    expect(readStoredTheme()).toBeNull();
+  });
+
+  it('applies a theme to the page and remembers it', () => {
+    applyTheme('dark');
+    expect(document.documentElement).toHaveAttribute('data-theme', 'dark');
+    expect(readStoredTheme()).toBe('dark');
+  });
+
+  it('still applies the theme when storage is blocked', () => {
+    const original = Storage.prototype.setItem;
+    Storage.prototype.setItem = () => {
+      throw new Error('blocked');
+    };
+    try {
+      expect(() => applyTheme('light')).not.toThrow();
+      expect(document.documentElement).toHaveAttribute('data-theme', 'light');
+    } finally {
+      Storage.prototype.setItem = original;
+    }
+  });
+
+  it('gives the head script that sets the saved theme before first paint', () => {
+    localStorage.setItem(THEME_KEY, 'dark');
+    new Function(themeInitScript)();
+    expect(document.documentElement).toHaveAttribute('data-theme', 'dark');
+  });
+
+  it('leaves the page on the device theme when nothing is saved', () => {
+    new Function(themeInitScript)();
+    expect(document.documentElement).not.toHaveAttribute('data-theme');
+  });
+
+  it('treats a browser without matchMedia as light instead of crashing', () => {
+    const original = window.matchMedia;
+    // @ts-expect-error simulating an environment that lacks matchMedia
+    window.matchMedia = undefined;
+    try {
+      expect(currentTheme()).toBe('light');
+    } finally {
+      window.matchMedia = original;
+    }
+  });
+});