Fix viewUpdateNSBG: re-declare lastBGTime local (#595)

Restore `let lastBGTime = entries[latestEntryIndex].date` in
viewUpdateNSBG. Dev currently fails to compile with
"Cannot find 'lastBGTime' in scope" because two PRs that merged
into dev on the same day textually did not conflict but semantically
interfered:

- #537 (Live Activity) added
  `Storage.shared.lastBgReadingTimeSeconds.value = lastBGTime`
  later in the function, relying on the existing local.
- #565 (Display HIGH/LOW) removed the local declaration as part
  of an unrelated refactor; that branch was cut before #537 landed,
  so from its perspective the local was unused.

Git's 3-way merge produced a syntactically clean file but the
surviving reference now dangles. Re-adding the local is the
minimal, intent-preserving fix.

Author: bjorkert

LoopFollow/Controllers/Nightscout/BGData.swift

@@ -236,6 +236,7 @@ extension MainViewController {
             let latestBG = entries[latestEntryIndex].sgv
             let priorBG = entries[latestEntryIndex - 1].sgv
             let deltaBG = latestBG - priorBG
+            let lastBGTime = entries[latestEntryIndex].date
 
             self.updateServerText(with: sourceName)