fix(web): downgrade Next.js 16.2.6 -> 15.5.18 to unblock Cloudflare deploy

The first auto-deploy from PR #3855 returned HTTP 500 on every page
with the runtime error `TypeError: components.ComponentMod.handler is
not a function`. Captured via `wrangler tail`.

Root cause: Next.js 16.2.6 was published 2026-05-07 19:01 UTC, *after*
@opennextjs/cloudflare 1.19.8 was published earlier the same day at
11:33 UTC. OpenNext 1.19.8's peerDependency declares
`next: '>=15.5.16 <16 || >=16.2.5'` — 16.2.6 falls inside the range
syntactically, but the route component module export shape changed in
that patch and OpenNext has not caught up yet.

Pin Next + eslint-config-next to 15.5.18 (latest 15.x LTS, the other
half of OpenNext's supported range). Revert the migration-only changes
that came with the 16 bump:

- eslint.config.mjs: `nextPlugin.configs["core-web-vitals"]` (v16
  shape) -> `nextPlugin.flatConfig.coreWebVitals` (v15 shape).
- tsconfig.json: `jsx: "react-jsx"` -> `jsx: "preserve"` (Next 15
  default).
- tsconfig.json: add `noUncheckedSideEffectImports: false` because
  TypeScript 6 enabled this option under `strict` and Next 15's
  bundled types do not declare ambient CSS modules (Next 16 does).

All other web/ deps stay at latest. lucide-react remains pinned at
0.577.0 from #3853 for the same brand-icon reason. Re-evaluate Next 16
when @opennextjs/cloudflare ships a release explicitly tested against
\>= 16.2.6.

web/eslint.config.mjs

@@ -18,7 +18,7 @@ export default [
       },
     },
   },
-  nextPlugin.configs["core-web-vitals"],
+  nextPlugin.flatConfig.coreWebVitals,
   ...tseslint.configs.recommended,
   prettier,
   {