fix(web): say which page is current, not just colour it

[42-v]

UI plan item A5, the half that needs no new strings.

The nav already knew. isActive(link.to) picks the highlight at all three link sites, and the result was expressed entirely as a background tint and a text shade — so a screen-reader user tabbing the navigation heard the same thing on every entry and could not tell which page they were on. WCAG 1.3.1: what presentation conveys has to be available programmatically too.

aria-current="page" now comes from the same call that picks the colour, so the two cannot disagree. Bound to undefined rather than "false" on inactive links — aria-current="false" is a value assistive technology reads and then ignores, which is noise where absence is the correct signal.

Site	Where
desktop nav loop	App.vue:57
signed-out sign-in link	App.vue:82
mobile nav loop	App.vue:123

The gate pairs them rather than checking a list of links: any element whose appearance consults isActive must also bind aria-current. That holds the fourth nav entry somebody adds, which a component test of the three that exist today would not. Mutation-verified — removing the binding from any one site fails it by name:

web/src/App.vue: an element decides its appearance from isActive() but binds no
aria-current, so the current page is signalled by colour alone.

Not in this PR: the skip link and <main id tabindex>, the other half of A5. That needs common.skipToContent across 38 locales, which is the one new string in the whole UI plan and an owner decision (#297).

Verified: 749 web tests pass; pnpm lint clean; branches 707/715 → 713/721, the new ternaries all covered; statements unchanged.

fix(web): say which page is current, not just colour it