fix(harmonyos): adapt welcome sheet to system insets

[wgqqqqq]

Summary

• make the compact HarmonyOS welcome layout fill the available height instead of reserving a fixed bottom gap
• draw the compact welcome sheet through the system gesture area with a platform window adapter, while restoring normal window layout for wide and foldable surfaces
• add dedicated welcome light/dark surface tokens so dark mode keeps a clear sheet/background boundary

Validation

• pnpm run mobile:ui:check
• pnpm run harmony:architecture
• pnpm run theme:color-audit:all
• HarmonyOS HAP build with hvigor ... assembleHap --no-daemon
• Installed and checked on the connected HarmonyOS device in light and dark appearances