caption only ever spawns whisper, but doctor probes three names, so doctor can be green while caption fails #146

Summary

doctor and caption resolve the Whisper binary differently:

• src/doctor.ts — onPath("whisper") ?? onPath("whisper-cli") ?? onPath("faster-whisper")
• src/caption.ts — const cmd = opts.whisperCmd ?? "whisper"; and, with no
  explicit --whisper-engine, detectEngine() assumes the openai dialect

So on a host whose only Whisper is whisper.cpp — a whisper-cli binary and no
whisper — doctor reports the check as ok, and the first caption run fails
with ENOENT while the error text points at --whisper-engine, not at the command
name that could not be found. whisper-cli is the name whisper.cpp has shipped
since v1.7.4 (previously main), and what Homebrew's whisper-cpp installs, so
this is the common case, not an exotic one.

Reproduction

Any host with whisper-cli on PATH and no whisper:

$ capcut doctor -H
[✓] whisper            found: /opt/homebrew/bin/whisper-cli

$ capcut caption ./my-draft --audio ./voice.wav --language zh
# → whisper CLI failed: spawnSync whisper ENOENT
#   Tried: whisper ./voice.wav --model base --language zh ...
#   Select the matching dialect with --whisper-engine openai|whisper-cpp|faster-whisper

Note the hint: the user is told to pick a dialect, but the actual problem is that
whisper was never the installed command name. The same mismatch hides a second
bug — even when the command is found by some other route, dialect detection
still reads the unresolved default, so a whisper.cpp binary would be driven with
openai's argument shape.

Measured on Windows 10.0.19045 with whisper.cpp's whisper-cli.exe; the same
resolution order is what a caller has to work around with
--whisper-cmd <path> --whisper-engine whisper-cpp today.

Expected

One resolver used by both, so doctor's answer is the answer caption obeys:
--whisper-cmd wins, then the first of whisper / whisper-cli /
faster-whisper actually on PATH, then the historical default — and dialect
detection reads whatever that resolved to.

Proposed fix

src/whisper.ts owns the PATH lookup and the preference list; doctor and
caption both call resolveWhisperCommand(). Explicit flags keep priority. The
resolver accepts an injectable probe so the order is tested without a real
Whisper install.

Patch: submitted as a PR from the same batch.