Mobile-optimized session UI

[Bryan-nyx]

Priority: P0
Estimate: 1.5 days

• One-handed phone session view
• Large timer
• Reachable answer buttons
• Reachable confidence control
• Full-width submit button
• No scroll during answer phase

Acceptance criteria:

• Entire answer UI visible without scrolling on 375px x 667px
• Tap targets >= 48px
• Timer readable at arm's length
• No accidental submissions from thumb proximity