Walkthrough artwork has English UI copy baked in (wt-3, wt-6)

[AndreaDiazCorreia]

Two of the walkthrough illustrations have English UI copy baked into the artwork, so users on es / fr / de / it read localized slide text next to hard-coded English inside the image:

• assets/images/wt-3.webp — "Buyer", "Seller"
• assets/images/wt-6.webp — "New offer", "Amount", "Payment method"

The other four slides are text-free (wt-1.webp shows the "Mostro" wordmark, which is a brand name and fine).

This is inherited, not a regression: the assets are byte-identical to the ones MostroP2P/mobile ships today, and they were brought over in #264 to replace the 817-byte placeholder rectangles that had no text at all. Parity was the right call there, but the localization gap should be closed.

Related: the drawer header (assets/images/mostro_logo_beta.webp) has "Beta" baked in, which regresses French — app_fr.arb has "betaBadgeLabel": "Bêta" and the previous header rendered it as localized text. Same root cause: user-facing copy inside raster assets.

Options

• Commission or generate text-free variants of wt-3 and wt-6 (preferred — one asset per slide, no per-locale matrix).
• Or ship locale-specific artwork via Flutter's asset variants, which means 5 copies of each affected image.
• Decide whether the drawer beta badge goes back to a localized text chip over the mascot, or stays as a single raster.

Affects lib/features/walkthrough/screens/walkthrough_screen.dart and lib/features/drawer/screens/drawer_menu.dart.