fix(web): a11y essentials + restore broken benchmark viewMode

[frapercan]

Why

UX audit flagged: only 3 of 14 pages used aria/role/label htmlFor. Drive-by: when PRs #81 (heatmap) and #82 (URL-sync) auto-rebased onto each other, the viewMode state on /benchmark was dropped — develop currently fails to build that page. Folded the trivial fix into this PR so it can land.

What

A11y

• Skip-to-content link in the locale layout: hidden until focused, lands focus on <main id="main" tabIndex={-1}>. Reachable on the first Tab press.
• NavLinks dropdowns now expose aria-haspopup="menu", aria-expanded, aria-current="page" on active groups. Inner items carry role="menuitem". Mobile menu locks body scroll while open and restores prior overflow on close (no more dual-scroll trap).
• StatusBadge gains a redundant leading glyph per state (clock / pulse / check / ✕ / slash-circle). Color-blind users still parse the badge at a glance. Wrapper carries role="status" and a verbose aria-label.
• Benchmark filter chips (Pipeline stage, Neighbours K) gain aria-pressed plus role="group" with descriptive aria-label.
• Heatmap | Table toggle already ships role="tablist" / aria-selected from feat(web): benchmark heatmap small-multiples — visual ranking per cell #81.

Drive-by build fix

• Restore viewMode state and the BenchmarkHeatmap import on /benchmark. PR feat(web): benchmark heatmap small-multiples — visual ranking per cell #81's toggle UI references viewMode / setViewMode, but the auto-rebase ladder against feat(web): URL-sync filters + sticky table headers + scroll-shadow indicators #82 dropped the hook line on develop, leaving the page broken at build time. Adds the lines back; default "heatmap".

Test plan

• next build green; 18 routes; no TS errors (was failing on develop before this PR)
• Backend untouched
• Press Tab on first load — "Skip to main content" link appears top-left, focusing it jumps to <main>
• Use a screen reader on /jobs — StatusBadge announces "Status: succeeded" etc.
• On a small viewport: open the mobile menu, body underneath stops scrolling; close and underneath scrolls again
• Tab into Pipeline-stage chips — focus ring appears, aria-pressed flips on selection