#57 build: bump Node version, update tools

Summary by cubic

Updates the build toolchain to Node.js 24, Yarn 4.18.1, TypeScript 6, tsdown 0.23, and web-ext 10, and migrates the tsdown config to the new rolldown/tsdown API.

• CI workflows now use Node.js 24, and docs bump the required LTS version accordingly.
• tools/tsdown_config.ts was rewritten for tsdown 0.23: outputOptions, deps.alwaysBundle, and codeSplitting.groups replace the old noExternal/advancedChunks options, and replacePlugin now comes from rolldown/plugins.
• JSON imports use with { type: "json" }, local imports use .ts extensions, and rolldown is pinned to ^1.2.14 instead of latest.
• tools/tsconfig.json enables allowImportingTsExtensions and emitDeclarationOnly.