Standalone: persist the requested sample rate, and validate it on an input-only stream

[defiantnerd]

Closes #555. Two commits, independently reviewable.

1. Persist what the user asked for. captureAudioSettings() wrote back the resolved rate, so saving 96 kHz and launching once on a 48 kHz-only device lost 96 kHz permanently — the same failure SA-2 (5ab316e) fixed for device names. Sample rate now follows that established intent-versus-runtime split: settings.sampleRate is written only at the point of choice (the frontend), and the resolved rate stays in currentSampleRate.

2. Validate on the input-only path. SA-3 clamped bufferSize because the .conf is hand-editable; sample rate was left to startAudioThreadOn, which only validates it when output is in use. Input-only streams passed it straight to openStream(). Both branches now share a rateOfferedBy() helper.

While there, a latent bug this makes reachable: the last-resort guard was sampleRate < 0, but 0 is the "device preferred" sentinel and the StandaloneSettings default. The output path caught 0 incidentally (0 is never in a device's rate list, so it fell through to preferred); input-only did not, and sent 0 to openStream(). Now <= 0. This matters more after commit 1, since fresh configs now carry 0.

Scope note. standalone_host_audio.cpp is touched because startAudioThreadOnImpl lives there — the validation cannot be done from applyAudioSettings(), since the frontends call startAudioThreadOn directly with an arbitrary rate.

Windows. Two single statements in windows_standalone.cpp: persisting the chosen rate in the AudioSamplerate handler (mirroring the existing device-name handlers), and a refreshSampleRates() after startAudio() — without it, commit 1 leaves a fresh .conf showing combo index 0 while the stream runs at the device's preferred rate. Not compilable on macOS; CI is the first thing to build it.

Build-gated on clap-first-distortion_standalone (which needs STANDALONE_CONFIGURATIONS enabled locally — not part of this PR), clean.