Show what's new after an update

[afitzgerald]

What

• What's New dialog: shows once after an update, listing every release since the one last seen, grouped New / Fixed / Other changes. Help → What's New in Diffier opens the whole baked history (disabled in local builds, which have no notes). Rebindable action whats-new; Escape closes it.
• Parser: main/whats-new.ts (zero deps) strips the by @author in <url> tail, New Contributors and Full Changelog, and compares versions numerically. "Seen" is whatsNewSeen in settings, keyed on app.getVersion(); the first window to boot marks it, so only that window shows the dialog.
• CI: release.yml runs scripts/release_notes.sh right after the release is created, writing that release's notes plus the nine before it into WhatsNew.md, which package.json files ships. Baked rather than fetched: works offline and always matches the build.
• .github/release.yml: groups PRs by label; internal and documentation are left out. The internal label now exists.
• CLAUDE.md: Pull requests are release notes: user-facing titles, exactly one label. Commit subjects stay conventional; next_version.sh reads those, not PR titles.

Checked

• yarn typecheck, yarn test (new test/whats-new.test.ts: parsing, numeric order 0.0.10 > 0.0.8, fresh install, all-internal release dropped) and node test/ui.test.js all pass. shellcheck is clean on the script.
• scripts/release_notes.sh v0.0.7 against the real releases parses to the expected three entries.
• Launched the app with a sample WhatsNew.md: the dialog appeared at boot and whatsNewSeen was recorded.

Not checked

• The first real release after merging: that the bake step runs in CI and the packaged app picks up WhatsNew.md from the asar.
• Tests and typecheck were not re-run after the rebase onto docs: add MIT license #83, which only adds LICENSE and README lines.