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/public/icons/README.md

@@ -1,3 +1,5 @@
 Single-colour line icons exported from the Figma file "Wursor - weown.net" (Icon/* components).
 Used through the `Icon` component (src/ui/Icon.tsx), which paints them with CSS masks so the colour comes from the
 current theme. Do not edit the files; re-export from Figma if an icon changes.
+
+Exception: `sun.svg` and `moon.svg` (theme toggle) are not Figma exports. They are hand-drawn to match the set (16px, 1.5 stroke, round caps). Replace them with the Figma Icon/Sun and Icon/Moon exports when those exist.

web/public/icons/moon.svg

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+<svg preserveAspectRatio="none" overflow="visible" style="display: block;" width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
+<g id="Icon">
+<path id="Vector" d="M14 8.52667C13.8828 9.79559 13.4066 11.0049 12.6277 12.0136C11.8489 13.0222 10.8001 13.7881 9.60331 14.2216C8.40652 14.6551 7.11116 14.7384 5.86854 14.4617C4.62591 14.1849 3.48765 13.5596 2.58652 12.6585C1.68538 11.7574 1.06015 10.6191 0.783359 9.37649C0.506571 8.13387 0.589915 6.83851 1.02339 5.64172C1.45687 4.44493 2.22275 3.39615 3.23142 2.61732C4.24009 1.83849 5.44941 1.36227 6.71833 1.24503C5.97543 2.25034 5.61787 3.48847 5.71073 4.73497C5.8036 5.98147 6.34073 7.15267 7.22402 8.03597C8.10732 8.91927 9.27852 9.4564 10.525 9.54926C11.7715 9.64213 13.0097 9.28457 14.015 8.54167L14 8.52667Z" stroke="#14120B" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+</g>
+</svg>

web/public/icons/sun.svg

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+<svg preserveAspectRatio="none" overflow="visible" style="display: block;" width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
+<g id="Icon">
+<path id="Vector" d="M8 11.3333C9.84095 11.3333 11.3333 9.84095 11.3333 8C11.3333 6.15905 9.84095 4.66667 8 4.66667C6.15905 4.66667 4.66667 6.15905 4.66667 8C4.66667 9.84095 6.15905 11.3333 8 11.3333ZM8 1.33333V2.66667M8 13.3333V14.6667M2.28667 2.28667L3.23333 3.23333M12.7667 12.7667L13.7133 13.7133M1.33333 8H2.66667M13.3333 8H14.6667M2.28667 13.7133L3.23333 12.7667M12.7667 3.23333L13.7133 2.28667" stroke="#14120B" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+</g>
+</svg>