fix(updates): resolve dream-update.sh path so dashboard updates actually work

[boffin-dmytro]

Summary

• The /api/update endpoint in the dashboard API tried the same wrong path twice (INSTALL_DIR.parent/scripts/dream-update.sh) and never checked the actual script location at INSTALL_DIR/dream-update.sh
• This meant the dashboard update, check, and backup actions would always fail with "Update system not installed" on a standard deployment
• Fixed the resolution order to check the correct path first, then fall back to legacy locations

Test plan

• Verify dream-update.sh exists at INSTALL_DIR/dream-update.sh on a standard install
• Trigger a version check from the dashboard — should no longer return 501
• Confirm the fallback paths still work for non-standard installations