Make docs search respond to arrow keys, and the mobile menu opaque

[fstubner]

The last two findings from the independent acceptance pass.

1. Arrow keys did nothing in the docs search

Pagefind's default UI leaves focus in the input and ignores ArrowDown/ArrowUp, so results were reachable only by tabbing. Tab and Enter did work — search was usable — but the down arrow is the key most people try first, and it appeared to do nothing at all.

Focus moves for real rather than painting a highlight and tracking aria-activedescendant. The results are anchors, so focus gets Enter, middle-click and screen-reader announcement for free, and there's no second notion of "selected" to keep in sync with the rendered list.

Results are re-queried on every keypress — Pagefind re-renders the list as the query changes, so any cached node would be stale by the next keystroke.

Down from the input enters the list; up from the first result returns to the input, so the query is one keypress away rather than Shift+Tab. Up from the input wraps to the last result.

Verified on the deployed build:

start -> INPUT
Down  -> result: Packet capture
Down  -> result: Packet capture
Down  -> result: CLI Capture
Up    -> result: Packet capture
Up    -> result: Packet capture
Up    -> INPUT

2. The mobile docs menu was see-through — and dark in light theme

The panel background was a hard-coded rgba(27, 32, 40, 0.96), which failed twice over:

• the 4% let the page read through the open menu, with body text crossing the menu labels
• because the colour was hard-coded rather than tokenised, the panel stayed dark in light theme — a dark drawer over a #fbfbfb page, with white showing through it

Now var(--sl-color-bg-sidebar) — #111 in dark, #fbfbfb in light, opaque in both. Measured after the fix; the panel follows the theme.

One sub-finding did not reproduce

The same report noted the first sidebar group label sitting behind the breadcrumb bar. That was the breadcrumb showing through the translucent panel, so the opacity fix resolved it. Confirmed with elementFromPoint: the panel's own content is topmost across its full area, nothing overlaps.

Verification

astro check clean, build passes, a11y gate 14 routes × both themes 0 violations, file-size guard clean.

Worth noting: the arrow-key behaviour was verified against the deployed build only after a cache-bust — the browser had held stale HTML referencing a previous script hash, which made the fix look like it wasn't working. The CDN was correct throughout.