Show loading spinners next to each instrument value during conditions fetch

- Add _conditionsLoading StateFlow to MainViewModel; set true when
  loadConditions() fires, false when the fetch completes (success or fail)
- Cancel any in-flight conditions job when a new loadConditions() call
  arrives (e.g. rapid map pans) so only the latest fetch drives the state
- Add a 10dp ProgressBar after each of the six data values in
  layout_instruments_sheet.xml (wind, temp, baro, current, waves, swell);
  visibility GONE by default, alpha 0.5; header section tinted
  colorOnSurfaceVariant, forecast section tinted #5B9EC2 to match its labels
- Observe conditionsLoading in MainActivity and toggle all six spinners
  together (all values come from one API call so they load in sync)

https://claude.ai/code/session_01HXPjBsogsJVRwCiekDGkJX

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