Modernize build

[sixem]

Addresses some build/tooling concerns raised in #57.

• Fix webpack configuration on modern Node versions; verified builds and tests on Node 20, 22 and 24.
• Switch to pnpm with a pinned version and lockfile; update compatible dependencies.
• Expand linting to all TypeScript, SCSS and build scripts, and fix existing findings.
• Remove unused Babel tooling and obsolete loaders.
• Define browser targets, improve CSS/font processing and fix development source maps.
• Fix standalone generation with nested asset paths and add regression tests.
• Make release packaging repeatable and non-destructive, with automated packaging checks.
• Remove obsolete Travis configuration.
• Refresh build instructions, automate the banner year and bump the version.