Fix sleep data parsing to match actual API response

[talison]

Summary

This PR addresses two issues with the sleep commands:

1. Range command flag parsing (sleep_range.go): The --from and --to flags were being read via viper.GetString() which wasn't receiving the flag values properly. Changed to read directly from cobra's cmd.Flags().GetString().

2. SleepDay struct field mapping (eightsleep.go): The struct fields didn't match the actual API response structure. The API returns:

   • sleepDuration (not sleepDurationSeconds)
   • deepDuration, remDuration, lightDuration for sleep stages
   • sleepStart, sleepEnd timestamps
   • Nested sleepQualityScore with hrv.current, heartRate.current, etc.

Changes

Updated the struct and commands to properly parse and display:

• Sleep duration in hours (more readable than seconds)
• Deep/REM/Light sleep durations
• Sleep start/end times
• RHR (resting heart rate)
• HRV
• Respiratory rate

Before/After

Before:

date        score  duration  tnt  resp_rate  heart_rate  hrv_score
2026-01-30  94     0         9    0          0           0

After:

date        score  duration_hrs  deep_hrs  rem_hrs  light_hrs  rhr  hrv   resp_rate  tnt
2026-01-30  94     7.1           1.5       1.9      3.5        57   54.9  15.3       9

Test plan

• Tested eightctl sleep day against live API
• Tested eightctl sleep range --from X --to Y against live API
• Verified JSON output format

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