Sync button extrainfo

Improvements

• Stable and correct separation between Redmine tooltip and native browser tooltip on the sync button.
• Redmine tooltip now consistently shows the localized “Update” label.
• Native tooltip displays dynamic, localized “Last synced …” information (seconds/minutes/hours/days).

Technical

• Prevents race conditions between hover handling and periodic tooltip updates.
• Tooltip logic is robust against auto-refresh and long-lived sessions.
• No impact on other tooltips or Redmine core behavior.

Compatibility

• Fully backward compatible.
• No data migration required.