chore(deps): typescript 5→6, @types/node 22→25, esbuild 0.25→0.28

All three were drop-in within the major-version range; the only
required code change was adding `src/types/css.d.ts` to declare the
`*.css` side-effect import shape (TypeScript 6 stopped silently
accepting unknown side-effect imports).

Tests: 1185/1185 vitest. tsc clean. build:worker clean.

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

src/types/css.d.ts

@@ -0,0 +1,8 @@
+/**
+ * TypeScript 6 enabled stricter side-effect import checking and stopped
+ * implicitly accepting CSS imports. Declare them here so `import './globals.css'`
+ * keeps type-checking under TS6.
+ */
+declare module '*.css';
+declare module '*.scss';
+declare module '*.sass';