feat(mobile): add gallery QR import and onboarding assets

[snhsish]

Added:

• Gallery picker button
• QR image resizing
• JPEG decoding via jpeg-js
• QR decoding via jsqr
• Shared helper at apps/mobile/lib/qr-from-gallery.ts
• Required dependencies and lockfile updates

Onboarding changes:

1. Assets → WebP @1200×1200 (delete PNGs)

• Add sharp devDep to apps/mobile, add scripts/optimize-assets.mjs that resizes onboarding_1_dark/light.png → onboarding_1_dark/light.webp @1200×1200 (q≈80).
• Run it; delete the two 2000×2000 PNGs; remove sharp from devDependencies.

2. apps/mobile/app/onboarding.tsx

• Use Image from expo-image; require(...webp); contentFit="contain", transition={250}, and a theme-matching tile bg so nothing flashes blank.