docs: clarify that serial_number field requires device Unit ID

[jat255]

Updates documentation and UI prompts to explain that Garmin's FIT file "serial_number" field must contain the device's Unit ID (found in Settings → About → Copyright Info) rather than the actual serial number. This distinction is critical for Garmin Connect to properly recognize activities and apply Training Effect calculations.

Changes:

• Add visual guides showing where to find Unit ID on Android app and Edge devices
• Update all references from "serial number" to "Unit ID" in critical sections
• Clarify that FIT file serial_number field stores Unit ID, not Serial Number
• Update config prompts to emphasize Unit ID vs Serial Number distinction
• Add note that Garmin Connect method may not work for all device types

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (6192294) to head (d595f51).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #68   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines         1235      1237    +2     
=========================================
+ Hits          1235      1237    +2     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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