Start scroll mode at bottom of scrollback by default

[guysmoilov]

Problem

When opening scroll mode, starting away from the latest output makes navigation awkward on mobile.

Proposal

Initialize scroll mode at the bottom (most recent content), then allow upward navigation from there.

Scope

• Enter scroll mode with viewport anchored to latest line
• Preserve expected behavior for manual upward scrolling after entry
• Ensure this does not break existing compose/input interactions

Acceptance Criteria

• Entering scroll mode shows latest output first
• User can immediately scroll upward to older content
• Behavior is consistent across reconnect/state refresh
• Add/update tests that assert initial scroll position on entry