fix(transcription): resample from the real capture sample rate

[farce1]

What

The transcription worker hard-coded a 48 kHz → 16 kHz resampler (worker.rs), but capture falls back to the device's native rate when it can't force 48 kHz F32 (capture.rs select_mic_config). The most common real case: Bluetooth headset mics in HFP mode deliver 16 kHz (AirPods etc.). Those 16 kHz samples were resampled as if 48 kHz → ~3× speed/pitch distortion → effectively garbage transcripts for a large slice of meeting users. The Opus-encoded OGG already used the real rate, so only the transcription path was wrong.

Fix

• RecordingState now carries transcription_sample_rate, set from the stream that actually feeds ASR: mic rate in Mic/Both modes, loopback rate in System mode (transcription_source_rate helper).
• Plumbed through StartWorkerArgs → WorkerConfig → the worker's AudioResampler::new(real_rate, 16k, …) (with a zero-guard fallback to 48 kHz).
• The 48 kHz path is byte-identical; only non-48k devices change (now correct).

Verification (TDD)

• RED→GREEN: transcription_source_rate tests — demonstrated RED by mimicking the old hard-coded 48k (tests failed left: 48000, right: 44100), then GREEN with the real selection.
• Added resampler tests proving correct handling of 16 kHz and 44.1 kHz inputs.
• Rust gate locally: clippy --all-targets --all-features -- -D warnings ✅; full cargo test → 5 new tests pass, only the pre-existing model_archive_consts_tests (REPLACE_WITH_ placeholders) fail, unrelated.

Note on interaction with #5

With #5 (Both-mode mic+system downmix) also merged, Both mode resamples at the mic rate; if mic and system rates differ, the system half is mis-resampled — the same pre-existing mixed-rate limitation flagged in #5. The common case (both 48 kHz) is correct. A fuller fix is canonical-rate normalization at capture (a larger, riskier change worth a dedicated decision).

⚠️ CI will be red — not because of this PR

main is red on three maintainer-only release blockers (SHA256/pubkey REPLACE_WITH_ placeholders, uncommitted vendor/vc_redist.x64.exe, separate frontend failure). Verified locally with a temporary untracked vendor placeholder. Do not merge until release prep is complete. Staged Track-A fix from the Handy vs openNotes transcription review; code-only, no new deps.