fix: blank canvas on subgroup tab switch

[knkenko]

Summary

• When switching between subgroup tabs, panes remount with grid = null but the PTY is already active — ensurePty is a no-op and no new terminal:render event fires
• Canvas stays blank until the user types (which triggers PTY output → render event)
• Fix: on mount, if the PTY is already in activePtyIds, request a fresh snapshot via scrollTerminal(paneId, 0) to immediately populate the canvas

Test plan

• Create a workspace with 2+ subgroup tabs (split panes, then add a new subgroup)
• Switch between tabs — verify terminal content appears immediately (no blank screen)
• Verify switching back to the original tab also shows content immediately
• Verify scrollback position is preserved correctly

🤖 Generated with Claude Code