fix(site): visible focus on selected filters, arrow keys stay in code blocks, pager polish

[TMHSDigital]

Closes #397, #398, #399.

Changes (scripts/build_gallery.py, scripts/site/template.html.j2; gallery regenerated)

• site: keyboard focus ring is invisible on selected gallery filter buttons (orange outline on orange fill) #397, focus visible: selected kind, category and chip buttons get outline: 2px solid var(--on-select); outline-offset: -4px. That's a dark inset ring on the orange fill (#1a1b1e on #ff8c19 ≈ 7.8:1). Unselected buttons are unchanged.
• site: arrow keys navigate to the next gallery page while trying to scroll a wide code block #398, arrow keys:
  • README and run-command <pre> blocks get tabindex="0", so a click focuses them and the keyboard can scroll them;
  • the pager key handler also stands down while the last pointerdown landed in a pre/.code-lines/.codewrap that overflows horizontally.
• site: small polish — pager loses its → at 375px, '/' search hint on touch, unnamed landing inputs, gallery LCP image lacks fetchpriority #399, polish:
  • the pager name truncates in its own .pager-label span, so the ← / → stays visible at 375 px;
  • the first card image (the LCP) gets fetchpriority="high";
  • on (hover: none) the search placeholder drops "(press /)";
  • the gallery search and the two landing filter inputs get name/id, which clears the DevTools form-field issue.
  • Not done: the 150 ms startup layout in onScroll(), which was the reviewer's lowest item.

Evidence (live browser: Playwright against docs/ served locally, after build_gallery.py + build_site.py)

Check	Result
Tab from search onto the selected "All" button	:focus-visible true. Computed solid 2px rgb(26,27,30) at offset −4px on rgb(255,140,25). Element screenshot shows a clear dark inset ring
vertex-color-ao: click the run block, press →	Focus lands on #runCmd; URL unchanged
Same, with the block forced to overflow (2882 px)	→ scrolls it (scrollLeft 23); URL unchanged
Nothing focused (click the h1, no selection), press →	Navigates to /gallery/cross-version-property-delete/, as before
bmesh-gear at 375 px, all 4 pager links	Label truncated, arrow box fully inside the link; screenshot shows ← driver-wa… and shader-no… →
Gallery first card	fetchpriority="high"; desktop placeholder keeps "(press /)"
python tests/check_site_links.py	135 pages, 0 failures

Not exercised: the touch placeholder branch (no (hover: none) emulation). It is a single matchMedia check.

🤖 Generated with Claude Code

[TMHSDigital]

CI evidence before merge: all checks pass ( 13 pass ). Blender Smoke on PR head: Blender 5.2.2 LTS / Blender 4.5.14 LTS. Summaries (happy path; falsifiers) per leg: 143 passed, 1 skipped, 0 failed;143 passed, 0 skipped, 0 failed;141 passed, 3 skipped, 0 failed;136 passed, 7 skipped, 0 failed;