workouts JSON: numeric bodyweight + sessionDurationSeconds (#33, #36)

[vinistoisr]

Fixes #33 and #36. Both touch the same workouts list/show JSON output, so they're handled together to avoid a self-conflicting diff.

Problems

• JSON 'bodyweight' field inconsistent type: string in workouts list, number in workouts stats #33 — bodyweight is emitted as a quoted string ("175") in workouts list/show, but as a number in workouts stats. Same field, two types depending on subcommand.
• JSON 'sessionDuration' is string while 'caloriesBurned' / 'prCount' are numbers #36 — sessionDuration is a human phrase ("01 hours 06 minutes 01 seconds") sitting next to numeric caloriesBurned/prCount; you can't do arithmetic on it without parsing prose.

Fix

Add Post.MarshalJSON (decoding is untouched — the API still sends strings; only output is normalized):

• bodyweight → JSON number, or null when absent/unparseable (matches workouts stats).
• keep the human sessionDuration string and add a numeric sessionDurationSeconds beside it.

Example:

{
  "startedAt": "2026-06-11T03:14:12.665Z",
  "sessionDuration": "01 hours 06 minutes 01 seconds",
  "bodyweight": 175,
  "sessionDurationSeconds": 3961,
  ...
}

Design notes

• I chose null (not 0) for a missing bodyweight so it isn't mistaken for a real measurement; happy to switch to 0 if you'd prefer strict number-always.
• sessionDurationSeconds is additive — the existing sessionDuration string is preserved, so nothing breaks for current consumers. The duration parser returns null on any shape it doesn't recognize rather than guessing.

Tests

TestPostMarshalJSON_NumericFields, TestPostMarshalJSON_EmptyBodyweightIsNull, and a table-driven TestParseDurationSeconds (including empty/non-numeric/unknown-unit/odd-field cases). go test ./... and go vet ./... pass.

🤖 Generated with Claude Code

[DTTerastar]

Verified locally: build, go vet, go test ./... all pass. Checked the marshaled output directly — single bodyweight key emitted as a number, no duplicate from the embedded alias, and sessionDurationSeconds lands alongside the preserved human string. Decoding path is untouched and nothing round-trips our own output back into Post, so no break for existing consumers. Agreed on null over 0 for a missing bodyweight — 0 would read as a real measurement.