feat(ui): migrate route transitions to React ViewTransition on Next 16.3

[JNZader]

Next 16.3 removed experimental.viewTransition (vercel/next.js#96098) — and the reason matters: the flag was already inert. Nothing in the 16.2.x runtime read it (verified locally: the string appears only in config-schema/config-shared; zero startViewTransition in next/dist/client). The ::view-transition-*(root) cross-fade in globals.css was dead code. On top of that, the flag's removal is the sole reason the Next 16.3 bump (#249) fails type-checking — so this PR supersedes it.

The replacement is the official 16.3 model: <ViewTransition> ships in Next's bundled React canary (no react@canary install), and every <Link> navigation already runs inside React.startTransition, so boundaries activate on route changes with zero config.

What changed

• next: ^16.2.11 → ^16.3.0 (resolves 16.3.1)
• next.config.ts: inert flag removed
• components/page-transition.tsx (new): <ViewTransition enter="page-fade" exit="page-fade" default="none"> boundary. Includes a runtime passthrough guard because vitest resolves stable react@19.2.x, which lacks the export — without it, page-rendering tests crash on "Element type is invalid". In Next (dev/prod) the real boundary is used; verified the export exists in both bundled builds (react.development.js:878, react.react-server.development.js:596).
• 12 public page.tsx wrapped (admin/studio excluded; sobre-mi is a bare permanentRedirect and renders nothing — excluded with a comment instead of a dishonest wrapper).
• globals.css: (root) selectors → class-based (.page-fade), preserving the 160ms-out / 220ms-in timings; added ::view-transition { pointer-events: none }; reduced-motion now zeroes durations on all pseudo-elements.
• types/view-transition.d.ts (new): module augmentation — the canary API is not in @types/react@19.2.x. Marked FORWARD-RISK: when @types/react ships it natively this file gets deleted, not edited.

Tests

• tsc --noEmit: exit 0 (this is the check chore(deps): bump next from 16.2.11 to 16.3.0 in the next group across 1 directory #249 fails)
• biome check: exit 0
• vitest run: 562/562 passed (71 files, local box)
• No local next build per standing rule — the Vercel preview build is the build validation. The animation itself is only exercised by Next's bundled canary at runtime; verify the cross-fade on the preview deploy.

Reviewed (reliability lens, 1 sweep)

No BLOCKER/CRITICAL. 3 WARNING: (1) dead PageTransition wrapper on the sobre-mi redirect — fixed, page excluded; (2) @types/react forward-conflict in the augmentation — fixed with a delete-me comment; (3) the silent runtime guard — accepted, it exists for the stable-react test runner and the export is verified present in both Next 16.3.1 bundles.

Follow-ups (NOT in this PR)

• Directional navigation (<Link transitionTypes={['nav-forward'|'nav-back']}> + slide CSS)
• Shared-element morphs (project thumbnail → project detail hero) — the highest-value pattern for a portfolio per the official guide
• Delete types/view-transition.d.ts when @types/react ships ViewTransition