ci: add screenshot gallery GitHub Pages deploy

[Krupakar-Reddy-S]

Description

Problem or Issue being addressed

The screenshot generator (tests/screenshot_generator/generator.py) already renders all views for every detected locale and produces gallery READMEs with translation completion percentages. CI runs it on every push and saves the output as artifacts. But there's no public-facing gallery — translators on Transifex can't see their work rendered visually.

Solution

Adds a new workflow (.github/workflows/screenshots.yml) that:

1. Triggers on push to dev
2. Generates screenshots for all locales
3. Renames README.md to index.md and fixes internal links for clean directory URLs
4. Builds HTML via actions/jekyll-build-pages (renders markdown + images)
5. Deploys to GitHub Pages via actions/deploy-pages

The index page lists all detected locales with links to per-locale galleries. Each locale gallery shows translation completion percentage and all screenshot sections (Main Menu, Seed, PSBT, Tools, Settings, Misc Error views).

Additional Information

• GitHub Pages must be enabled by a maintainer: Settings > Pages > Source: "GitHub Actions". The workflow will not deploy until this is done.
• Once enabled, the gallery will be reachable at https://seedsigner.github.io/seedsigner/
• Tested on fork: https://krupakar-reddy-s.github.io/seedsigner/
• This is a separate workflow from tests.yml — keeps deploy logic isolated and only triggers on dev pushes, not PRs.

Screenshots

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This pull request is categorized as a:

• New feature
• Bug fix
• Code refactor
• Documentation
• Other

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Checklist

I ran pytest locally

• All tests passed before submitting the PR
• I couldn't run the tests
• N/A

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I included screenshots of any new or modified screens

Should be part of the PR description above.

• Yes
• No
• N/A

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I added or updated tests

Any new or altered functionality should be covered in a unit test. Any new or updated sequences require FlowTests.

• Yes
• No, I’m a fool
• N/A

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I tested this PR hands-on on the following platform(s):

• Raspberry Pi OS Manual Build
• SeedSigner OS on a Pi0/Pi0W board
• Emulator

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I have reviewed these notes:

• Keep your changes limited in scope.
• If you uncover other issues or improvements along the way, ideally submit those as a separate PR.
• The more complicated the PR, the harder it is to review, test, and merge.
• We appreciate your efforts, but we're a small team of volunteers so PR review can be a very slow process.
• Please only "@" mention a contributor if their input is truly needed to enable further progress.

• I understand

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[wolgwang1729]

Nice improvement overall. One optional enhancement: can we add SeedSigner branding (logo/header styling) to the generated Pages site so it feels consistent with the main website?

Deferring to maintainers on product value and prioritisation.