ci: add static feature self-tests that run on every PR

[oahsiao]

Summary

Adds a fast, dependency-free self-test that runs as a check on every PR, so modifying one thing can't silently break another (e.g. the keyboard shortcuts).

It is pure static analysis — no Electron, no DOM, no npm install — so it finishes in seconds on ubuntu-latest via Node's built-in test runner.

What it guards (5 checks)

1. Syntax — every src/**/*.js passes node --check.
2. No duplicate methods — a class can't define the same method twice. This is the exact bug that broke "Select Folder" + Alt+F (two _selectFolder methods; the later one silently shadowed the first). Verified: the pre-fix file had 2 defs, current has 1.
3. Hotkey contract — Ctrl+F/D/T/W, Alt+F, Alt+Down, Esc stay wired, Alt+F and Esc are handled before the "user is typing" guard, and the methods they call exist on Tab.
4. Button wiring — every data-action in index.html has a matching act() handler (no dead buttons).
5. Signature icon — stays an inline <svg class="gh-icon"> (guards the "GH" text regression).

Runs as a PR check

.github/workflows/selftest.yml triggers on pull_request + push to main, so it shows up as a required-style status check on PRs automatically. Run locally with npm test.

Files

• test/selftest.test.js — the suite (node --test)
• package.json — adds the test script
• .github/workflows/selftest.yml — the PR workflow

Extending

Add another test(...) block in test/selftest.test.js — the helpers (classesIn, tabMethodNames) are reusable.