fix(auth): honor expires_in and bound transient network retries in device login

[duyet]

Summary

Device login parsed expires_in but the poll loop never used it, and transport errors retried forever (anyr relay start auto-login hits the same loop). This change extracts a pure poll_action helper and rewrites poll_device_token to:

• Give up when wall-clock elapsed time reaches expires_in (floored at 60s)
• Give up after 5 consecutive transport errors
• Keep sleeping before each attempt (no busy-wait)
• Leave poll_device_token / login_via_device public signatures unchanged

Only src/auth.rs is modified. No new crates.

Closes #18

Checklist

• Version stays on 0.1.x (do not introduce 0.2 / 1.0)
• Do not auto-merge release-please PRs

Skip co-authors

• I skipped co-author trailers on purpose

  

[github-actions]

anyr size and startup

Startup is wall time for a cold anyr --version / anyr --help (median of 21 runs). Size is the stripped release binary, or the .wasm for the browser demo.

Size budget: anyr-linux-x86_64 must stay ≤ 4.0 MiB (4194304 bytes) stripped.

Asset	Kind	Size	--version median	--help median
anyr-darwin-arm64	native	2.7 MiB	2.61 ms	2.33 ms
anyr-darwin-x86_64	native	2.9 MiB	30.34 ms	28.92 ms
anyr-linux-arm64	native	2.6 MiB	0.64 ms	0.65 ms
anyr-linux-x86_64	native	3.0 MiB	1.09 ms	1.05 ms
anyr-windows-x86_64.exe	native	2.5 MiB	9.21 ms	9.24 ms
anyr.wasm	wasm	175.1 KiB	—	—

Budget check: ok

raw timings

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