fix(mobile): preserve scrollback on session switch

[almogdepaz]

summary

• request full broker prefill when switching terminal sessions
• preserve initial mobile fast/full preference behavior
• verify touch scrolling works immediately without opening the keyboard

root cause

Mobile fast mode used viewport-only prefill after every session switch. That discarded broker history, so touch scrolling had nowhere to go. Opening the keyboard shrank the viewport and created local scrollback, masking the issue.

verification

• bunx playwright test tests/e2e/session-switch.e2e.ts --project=iphone-se --project=iphone-14 --project=desktop (20 passed, 31 project-gated skips)
• bun run typecheck
• bun test (1,642 passed)

Closes #163

[almogdepaz]

Superseded by #179, which contains this PR’s commit plus the consolidated dev07 hardening and terminal-recovery work. Closing so review stays centralized.