fix(ui): give nav links distinct focus identities

[acamarata]

The failure

nself-org/web's Accessibility (WCAG AA) gate went red on main
(run 34751962108):

Error: /products: 2 links have no accessible name
(only 1 may fall back to the shared href)

Why it is this repo's bug, not web's

The commit that went red changed exactly one file — a docs page
(licensing/offline.mdx) that links to nothing on /products. The two offending
links come from the shared header and footer in this package.

/products is linked from three renderings, none of which carried a data-testid
or aria-label:

Location	File
Desktop nav	Header.tsx (navItems.map, hidden lg:flex)
Mobile nav	Header.tsx (navItems.map, inside the collapsible panel)
Footer link groups	Footer.tsx (group.links.map)

tests/keyboard/keyboard-nav.spec.ts identifies a link by
data-testid ?? aria-label ?? href and skips anything with
offsetParent === null. At desktop width the mobile panel is hidden, but the desktop
nav link and the footer link are both visible and both fall back to the bare href —
two "unnamed" links for one destination, which the test reports as a collapsed focus
identity. Its own comment records this exact failure mode from 2026-09-11, where three
links collapsed into one identity and tripped a false focus-trap.

That also explains why it looked intermittent: it is viewport- and
render-visibility-dependent, not random.

The fix

A test-only disambiguator on all three renderings: nav-desktop-<label>,
nav-mobile-<label>, footer-<group>-<label>.

Deliberately not an aria-label. That would satisfy the gate while making two
otherwise-identical links announce differently to screen readers, for no user benefit
— worse accessibility in exchange for a green check. data-testid is invisible to
assistive tech and is the identity the test prefers first, so it fixes the real
collapsed-identity problem without touching what users hear.

Nothing user-visible changes.

Verification

pnpm build in @nself-web/ui is clean, and all three expressions are present in the
built output:

"data-testid": `nav-desktop-${item.label}`
"data-testid": `nav-mobile-${item.label}`
"data-testid": `footer-${group.title}-${link.label}`

The authoritative check is web's a11y gate against a build that consumes this — web
clones this repo fresh in CI, so it picks the change up once merged.