Fix prompt bubble sizing and improve contrast

[eivindjonassen]

Summary

• Fit prompt bubbles to their content with an 80% maximum width and right alignment.
• Add hidden, non-interactive iOS text measurement to prevent the native selection view from collapsing bubble width. Exclude the measurement text from accessibility.
• Use a lighter grey prompt background and remove the YOU label and its spacing.
• Preserve full-width accommodation for large text.

Validation

• pnpm check passed: lint, typecheck, tests, and both Hermes exports.
• pnpm native:doctor passed all 18 checks.
• Native fingerprints match runtime 0.1.5.
• Published to preview for iOS and Android: https://expo.dev/accounts/omnicus/projects/o5s-code/updates/e2ca7027-2458-4d54-b0e8-0679e9a47db9
• Final short/long prompt appearance still needs phone confirmation.