Add a screenshots skill for the app store images

[luflow]

Adds .claude/skills/screenshots/ so refreshing the four shipped screenshots in appinfo/screens/ is repeatable instead of guesswork.

What's in it

scripts/seed_demo_data.py — writes the choir demo data (10 appointments, ~110 responses) straight into oc_att_appointments / oc_att_responses. Going through SQL rather than the UI is the point: it produces "everyone has answered" states without logging in as eleven users.

• Non-destructive — only deletes and re-inserts the reserved ID range 101-199, and is idempotent.
• Times are relative to the instance clock, so re-running months later still gives sensible data: rehearsals on Tuesdays, concerts on Saturdays, and one appointment running right now, which is what unlocks the live "Start check-in" button.
• Autodetects the Nextcloud and database containers; reads DB credentials from config.php.
• Also fixes what quietly spoils the shots: display names (the dev accounts have none, so the UI shows user1), whitelisted_groups plus a Conductor group (members of groups missing from the whitelist collapse into an "Others" row that reads like a data bug), and lang=en with locale=en_GB (lang alone leaves dates German inside an English UI).

SKILL.md — the parts that are judgement, not automation:

• The sizing lever: keep viewport x deviceScaleFactor at 1952x1344 (the size of the existing set) and trade one against the other to fit more content. Includes working start values per screenshot.
• Measure element heights with JS instead of guessing, and cut lists between entries.
• Two traps that cost real time: leftover hover state making a button look selected, and the check-in buttons rendering both sides coloured for a pending attendee — which reads as "marked absent" unless the visible slice also contains a decided row. The seed data arranges present/absent/pending among the first five alphabetical entries for exactly this reason.
• Generate thumbnails from the PNG, not the JPG, to avoid compressing twice.
• The info.xml rules that actually bite: the element order is a schema-enforced sequence (screenshot goes after repository), and replacing an image only needs a merge to main while adding an entry needs a release, because the URLs point at refs/heads/main but the entries are release metadata.

Verification

• seed_demo_data.py --dry-run and a full run both exercised against master-stable34-1; verified via the app's own API that the response summary shows Conductor / Sopran / Alt / Tenor / Bass with no "Others", and that check-in states come out 4 attended / 1 absent / 6 pending with all three visible in the first five rows.
• Frontmatter matches the existing skills (name + description only); script mode 100755 like the others.
• Branched from main; no app code touched, so appinfo/** is unchanged and the backend suite is unaffected.

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