fix: restore web build (defaultTheme type + missing @eslint/js dep)

[novincode]

What

CI on main went red after #14 was merged. Two root-cause fixes:

1. apps/web build failure (the hard error)

#14 ("Refactor theme import") dropped the explicit LexKitTheme annotation on defaultTheme. Without it, TypeScript infers a type that references Lexical's internal modules, which can't be named portably during next build:

./components/templates/default/theme.ts:3:14
Type error: The inferred type of 'defaultTheme' cannot be named without a
reference to '@/node_modules/lexical/LexicalEditor'. This is likely not portable.

Fix: annotate explicitly and import the type from the public @lexkit/editor entrypoint — matching the shadcn template, which was already correct.

2. Missing @eslint/js dependency (the ⨯ ESLint line)

base.js, next.js and react-internal.js in @repo/eslint-config all import @eslint/js, but it was never declared, so it isn't resolvable under pnpm's strict layout:

⨯ ESLint: Cannot find package '@eslint/js' imported from packages/eslint-config/next.js

Pre-existing and non-fatal (Next continued past it), but it's a genuine undeclared dependency. Declared at the eslint v9 major to match the other lint deps.

Verification

pnpm -F web build passes locally — all 24 pages prerender (including /docs/extensions/CollaborationExtension), no type error, ESLint loads cleanly.