feat: add packaging script for Chrome/Firefox extension artifacts

[Copilot]

What changed and why

Adds a packaging workflow so developers can generate versioned, installable browser-extension artifacts (Chrome/Edge .zip and Firefox .xpi) from the repo.

• Added scripts/build.sh to stage a clean set of runtime extension files and produce versioned .zip/.xpi artifacts.
• Added npm run package script for packaging-only (separate from compile-only npm run build).
• npm run build remains compile-only (esbuild), keeping CI and local dev builds lightweight and cross-platform.
• The packaging script checks for both python3 and zip availability with clear error messages before running.
• Uses a portable mktemp invocation (mktemp -d "${TMPDIR:-/tmp}/glyphy.XXXXXXXXXX") that works on both Linux and macOS/BSD.
• Documented prerequisites, commands, outputs, and installation steps in README.md.

Test plan

• Loaded extension as unpacked in Chrome
• Tested on at least 2–3 different sites
• Icon fonts (Material Icons / Font Awesome) still render correctly
• No console errors on affected pages
• npm run build compiles JS sources successfully without invoking packaging
• npm run package produces Chrome .zip and Firefox .xpi artifacts in ~/Downloads/glyphy/
• Packaging script fails gracefully with clear messages if python3 or zip are missing

Related issues

[Copilot]

Pull request overview

Adds a packaging workflow so developers can generate versioned, installable browser-extension artifacts (Chrome/Edge .zip and Firefox .xpi) from the repo, integrating this into npm scripts and documenting usage in the README.

Changes:

• Added scripts/build.sh to stage a clean set of runtime extension files and produce versioned .zip/.xpi artifacts.
• Updated package.json scripts to run packaging as part of npm run build, and added npm run package for packaging-only.
• Documented prerequisites, commands, outputs, and installation steps in README.md.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
scripts/build.sh	New packaging script that builds versioned Chrome/Firefox artifacts from an explicit file allowlist.
package.json	Wires packaging into npm run build and adds a packaging-only alias.
README.md	Adds build/package instructions, prerequisites, output locations, and install steps.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.