fix: restore mobile terminal scrolling

[almogdepaz]

summary

• force full history prefill when opening the first mobile session so scrollback is available
• derive touch-scroll sensitivity from the rendered terminal row height
• cancel touch momentum and return to latest output when opening the mobile keyboard
• add mobile regressions for issues Mobile terminal sometimes becomes unscrollable after switching sessions #163, Increase touch-scroll sensitivity for long terminal histories on mobile #181, and Opening the mobile keyboard should scroll to the latest terminal output #182

verification

• bun run typecheck
• bun test — 1,683 passed, 20 broker-dependent skips
• bunx playwright test — 78 passed, 117 expected project/broker-dependent skips
• focused regressions passed on iPhone SE and iPhone 14 Playwright projects
• git diff --check

closes #163
closes #181
closes #182