Refresh the outdated store visuals (Play + App Store)

The current images in the app stores are outdated. We can benefit from better and more engaging visuals (why not videos) that attract more people.

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## Refresh scope

The screenshots and graphics on both listings no longer match what the app actually looks like. They need to be re-shot and re-uploaded for **Google Play** and the **App Store**.

## Current state

Store **release notes** are version-controlled and pushed automatically — `whatsnew/whatsnew-bg` and `whatsnew/whatsnew-en-GB`, wired up at `.github/workflows/upload-playstore.yml:69` via `whatsNewDirectory`.

Store **images are not in this repo at all** and are not touched by CI. They are managed by hand in each console, which is why they have drifted. Worth fixing as part of this — see "Keep them reproducible" below.

## Sequencing — do this after the UI changes land ⚠️

Two open issues will change how nearly every screen looks. Screenshots taken before they merge will have to be re-shot:

- **#216** — edge-to-edge inset handling. Changes the top and bottom of all 16 Scaffold pages.
- **#219** — branded splash screen. Changes the launch experience, and a splash frame is a natural first screenshot.

**Take the screenshots after those two are merged and released.** Doing this first is wasted work.

## What is needed

Verify the current requirements in each console before producing anything — the required sizes change periodically.

**Google Play**
- Phone screenshots (min 2, up to 8) — use all 8
- 7" and 10" tablet screenshots if we want tablet listing presence
- Feature graphic, 1024×500 — this is the banner and it is currently the weakest asset
- App icon, 512×512
- Optional promo video (YouTube link)

**App Store**
- iPhone screenshots — 6.9" and 6.5" display sizes
- **iPad screenshots are required.** `ios/Runner.xcodeproj/project.pbxproj` sets `TARGETED_DEVICE_FAMILY = "1,2"`, so the app ships as iPhone + iPad and Apple will demand iPad assets.
- Optional app previews (video, max 30 s)

**Localisation** — the app ships Bulgarian and English (`lib/l10n/app_localizations_bg.dart`, `app_localizations_en.dart`) and the store listings already carry `bg` and `en-GB`. Produce **both language sets**; Bulgarian is the primary audience.

## What to showcase

Lead with what is distinctive about 5kmRun.bg rather than generic app chrome. Strongest candidates:

- **Barcode page** (`lib/barcode_page.dart`) — the thing runners actually open at the start line
- **Run history and personal records** (`lib/runs/`)
- **Charts / progress over time** (`lib/offline_chart/`) — visually the most striking screens
- **Events page** (`lib/events/`) — upcoming races
- **Profile** (`lib/profile.dart`)
- **Strava integration** — a recognised brand, good for trust

Use a real, populated account so the screens are not empty — test user ID `13731` reaches a logged-in state via "login with id". Check that no personal data of real runners is visible before publishing.

## Keep them reproducible

Consider committing the source assets (and any generation script) under a `store-assets/` directory, mirroring how `whatsnew/` already works. Screenshot generation can be largely automated with Flutter integration tests plus a device-frame step — but note CLAUDE.md's rule that integration tests are not currently wired into CI and should not be added without discussion. A local-only script is fine.

## Acceptance criteria

- [ ] Requirements re-verified in both consoles (sizes change over time).
- [ ] Play: 8 phone screenshots, new 1024×500 feature graphic, icon confirmed current.
- [ ] App Store: iPhone 6.9" + 6.5" **and** iPad screenshots.
- [ ] Both `bg` and `en-GB` sets produced.
- [ ] Screenshots taken from a build that includes #216 and #219.
- [ ] No real runners' personal data visible in any image.
- [ ] Both listings reviewed side by side for visual consistency.
- [ ] Source assets committed (or a documented reason not to).

## Notes for whoever picks this up

- **This is mostly a design/marketing task, not a code task.** The feature graphic and any promo video need real design judgement and should not be auto-generated. What *can* be automated is the screenshot capture and device framing.
- Uploading to the consoles requires store credentials — that part will need @etabakov.
- Do not add dependencies for screenshot tooling without confirming first.