Release v1.8.0

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Merge pull request #34 from bpauli/feat/per-set-duration-rest

feat: per-set duration and rest time for exercise sets

fix: retry auth requests on HTTP 429 and add token export/import (#33)

Wrap the auth HTTP client with RetryTransport so login requests
automatically retry on 429 (rate limit) and 5xx responses with backoff.
This fixes headless login failures when Garmin's OAuth endpoints are
rate-limited.

Add `auth export` and `auth import` subcommands to transfer credentials
between machines without re-authenticating. Export outputs a base64
string; import accepts it as an argument or from stdin.

Closes #30

fix: update garmin-trainer skill homepage URL

docs: add v1.3.1 changelog entry