fix: point the Help button at the canonical documentation URL

[nelsonduarte]

What

The Help ("?") button in the title bar opened https://nelsonduarte.github.io/PDFApps/#guide, whose target section no longer exists. This is shipped in 1.15.0, so every user on the current release has a dead Help button. It now points at https://pdf-apps.com/docs#first-steps.

Why it broke, verified rather than assumed

Reconstructed with git log -S and independently confirmed by the reviewer:

• b14d11d added the button and, in the same commit, the <section id="guide"> it pointed at plus its nav entry. The link worked when written.
• 6847858 rewrote the repo path from PDFApps-en to PDFApps two days later. That is the form that shipped.
• 350a419 replaced the single-page site with the five-page layout and deleted both the #guide section and its nav link. Nothing has re-added them.

The button worked for about two weeks and broke in the website redesign. The later migration to Cloudflare Pages was not the cause: it changed the canonical host but did not remove the anchor. Please do not re-attribute this to Cloudflare.

Why nobody noticed

The old host still answers 200 and serves the current five-page content, so there is no 404 to expose the failure. An unknown fragment is not an error in HTTP or in any browser, so the user silently landed at the top of a page without the section, with no status code, log, or crash to catch it.

The new target is the canonical URL the site itself declares, which also avoids the 308 the old host would issue.

Tests

tests/test_help_url_domain.py (new, 3 tests). Suite goes from 746 passed / 2 skipped on main to 749 passed / 2 skipped. Reviewer and QA both approved.

Known debt NOT addressed here

Recorded deliberately so it is not lost:

• Two mutations survive all three new tests, both user-visible: removing the button from the layout, and removing the _a11y call. QA left the behavioural test that kills these, and which also kills the wiring and URL mutants. app/window.py is at 0% coverage.
• The three new tests are source-text assertions. They break under an innocuous refactor: extracting the URL into a constant would break the regex. For the concrete bug they prevent this is the right shape, but they should be complemented by the behavioural test above.
• Silent failure confirmed by execution: if webbrowser.open returns False or raises, the click produces no feedback at all. Pre-existing, not introduced here.
• Seven packaging manifests still point at the retired host, and the tests scope to app/ so they do not see them: aur/pdfapps/PKGBUILD:7, aur/pdfapps/.SRCINFO:5, aur/pdfapps-bin/PKGBUILD:8, aur/pdfapps-bin/.SRCINFO:5, rpm/pdfapps.spec:7, snap/snapcraft.yaml:22, winget/nelsonduarte.PDFApps.locale.en-US.yaml:9, plus <url type="homepage"> and four screenshot URLs in the Flatpak metainfo.
• Unpublishing or redirecting the GitHub Pages site is the only way to repair the links already shipped in 1.15.0, which cannot be recalled. The reviewer confirmed it is feasible, that it does not affect the application IDs in the manifests, and that the redirect must target /docs rather than the root.

Scope

One commit. No tags, no merge, no version change (APP_VERSION remains 1.15.0).

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