fix: resolve local timezone to IANA name for sleep commands

[dtrinh]

Summary

• sleep day and sleep range fail with 400 BadRequest when using the default --timezone=local because time.Local.String() returns "Local", which is not a valid IANA timezone name
• Added resolveTimezone() helper that detects the system IANA timezone by reading the /etc/localtime symlink (macOS/Linux) or TZ env var
• Fixes both sleep day and sleep range commands

Root Cause

The existing code did:

tz := viper.GetString("timezone")
if tz == "local" {
    tz = time.Local.String() // Returns "Local", not "America/New_York"
}

The Eight Sleep API rejects "Local" as an invalid timezone value.

Test plan

• eightctl sleep day --quiet works without specifying --timezone
• eightctl sleep range --from 2026-02-14 --to 2026-02-21 --quiet works without specifying --timezone
• Explicit --timezone America/New_York still works
• go build ./cmd/eightctl compiles cleanly

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