Modernize dependencies (next 16.2, ts 6, next-plausible 4, @vercel/analytics 2)

Single consolidated bump that supersedes the seven open Dependabot PRs
(#11, #49–54) with one coherent change. Lifts framework majors to current
and includes the necessary code migration for next-plausible v4.

Major bumps:
- next 16.1.5 → 16.2.4
- typescript 5.9.3 → 6.0.3 (drop-in: tsconfig already TS-6 ready, no baseUrl)
- @types/node 24.10.1 → 25.6.0
- @vercel/analytics 1.6.1 → 2.0.1 (drop-in: bare <Analytics /> API unchanged)
- next-plausible 3.12.5 → 4.0.0 (migration: see below)

Minor/patch sweep (≥14 days old, respecting the dependabot cooldown):
react 19.2.5, @types/react 19.2.14, @next/mdx 16.2.4, tailwindcss 4.2.3
(exact-pinned), @tailwindcss/postcss 4.2.3, resend 6.12.2,
rehype-pretty-code 0.14.3.

next-plausible v4 migration: v4 uses Plausible script v2's site-specific URL
instead of the legacy data-domain attribute. Renamed
NEXT_PUBLIC_PLAUSIBLE_DOMAIN → NEXT_PUBLIC_PLAUSIBLE_SCRIPT_SRC and made
both the withPlausibleProxy wrapper and PlausibleProvider conditional on
the env var so local dev (no env) builds and renders cleanly. CSP unchanged
— script still served from plausible.io.

Post-merge: set NEXT_PUBLIC_PLAUSIBLE_SCRIPT_SRC in Vercel (URL from the
Plausible dashboard for vortex.dev), remove NEXT_PUBLIC_PLAUSIBLE_DOMAIN.

Verified: check:ci, build, typecheck, verify (15/15 passed).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: lwwmanning