Remove unused happy-dom, postcss, and autoprefixer dependencies

[gregpriday]

Three devDependencies are installed but never referenced anywhere in the codebase:

• happy-dom (^20.8.4) — Zero test files use // @vitest-environment happy-dom. All 154+ DOM tests use jsdom exclusively.
• postcss (^8.5.8) — No postcss.config.* file exists. Tailwind v4 with @tailwindcss/vite uses Lightning CSS internally for all CSS processing and vendor prefixing.
• autoprefixer (^10.4.27) — No references in any config or source file. Redundant with Tailwind v4's built-in Lightning CSS vendor prefixing.

None of these packages are imported, required, or referenced in any configuration file outside package.json. Removing them cleans up the dependency tree and slightly speeds up npm install.

Note: jsdom and @types/jsdom must be kept — jsdom is actively used as the vitest environment in 154+ test files.