Fix boxel-ui docs scroll behavior and other improvements

[burieberry]

Fixes scroll issues in the boxel-ui Freestyle docs, improves the icons grid, reworks the mobile nav into a proper drawer, and adds theme importing.

Scroll fixes

• Patch FreestyleMenu#scrollActiveItemIntoView so the nav's scroll-spy scrolls only the nav's own container instead of using scrollIntoView, which was hijacking the in-progress page scroll whenever the active menu item was off-screen.
• Set overflow: visible on the guide's CardContainer — its overflow: hidden made it the containing scroll box, so the sticky nav sidebar never stuck to the viewport.
• Set overflow: visible on .FreestyleGuide-content too: its vendor overflow: auto made it the containing scroll box for sticky descendants, but the page body is what actually scrolls (the column is never height-constrained), so nothing inside could stick. Also cap it with min-width: 0; max-width: 100% so wide content (tables, code) can never push the column past its row.

Mobile nav drawer

• The nav is now a modal drawer on small screens: closed by default (users land on scrollable content), opened via the hamburger.
• Translucent backdrop that dismisses on tap; selecting a menu entry also closes the drawer. Body scroll-lock applies only while the drawer is open.
• Slides in with a transform transition; full-height inset positioning replaces the old top: 70px magic numbers; the hamburger reflects state via aria-expanded.

Icons grid improvements

• Added a "Show All" toggle that removes the grid's max-height so the full icon set can be browsed without the inner scroll area (including on mobile, where the capped height is now a max-height).
• The grid header (filter, Show All, count) is sticky while scrolling through the icons, with a shadow that appears only while pinned (IntersectionObserver sentinel — no scroll listeners).
• Toggling Show All off scrolls back to the top of the icons section instead of stranding the page deep in collapsed content.
• Replaced the raw filter <input> with BoxelInput + FieldContainer, and swapped hardcoded hex colors for theme variables (var(--border), color-mix hover).

Themes

• Consolidated theme definitions into tcn-themes.ts / cs-themes.ts barrels and added new themes (Amethyst Haze, Sage Garden, Boxel Brand Guide, …); display names are derived from the export names (SoftPop → "Soft Pop").
• New "Import Theme" panel: paste a theme export (e.g. from tweakcn), and only its :root / .dark variable blocks are kept — @theme blocks and resets are stripped. Apply is disabled until a valid block is present; names are de-duplicated.
• Fonts: the static Google Fonts link now covers every family the built-in themes reference, and imported themes get their --font-* families loaded dynamically (one link per family, so unknown fonts fail silently to their fallback stacks).

Other polish

• Theme settings header: flattened the layout into three inline FieldContainers (Theme / Cycle Themes / Dark Mode) and dropped the floating-position media-query CSS.
• Small typography tweaks (font-size tokens for subtitle, descriptions, mobile sizes) and a themed even-row color for the API/CSS-vars tables.

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[github-actions]

Preview deployments

• Host staging preview

• Host production preview

• Boxel UI staging preview

Host Test Results

    1 files  ±0      1 suites  ±0   3h 11m 59s ⏱️ + 5m 32s
3 548 tests ±0  3 533 ✅ ±0  15 💤 ±0  0 ❌ ±0 
3 567 runs  ±0  3 552 ✅ ±0  15 💤 ±0  0 ❌ ±0 

Results for commit 4f0af7d. ± Comparison against earlier commit 11cfc87.

Realm Server Test Results

    1 files  ±0      1 suites  ±0   17m 30s ⏱️ +50s
1 880 tests ±0  1 880 ✅ ±0  0 💤 ±0  0 ❌ ±0 
1 959 runs  ±0  1 959 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 4f0af7d. ± Comparison against earlier commit 11cfc87.

[backspace]

I don’t think this is blocking and maybe just a side-effect of the preview deployment but switching between icons and components causes a full refresh for me: