Native: download whisper model on demand + surface real errors

Two native-only fixes behind the "transcription failed [object Object]"
report on the APK:

1. Model download. engineImpl.native.loadModel assumed the ggml .bin was
   already on disk (the downloader was a TODO), so on a real device it
   rejected — and since in-app recording is web-only, no model was ever
   present. Now loadModel fetches the model from Hugging Face
   (ggerganov/whisper.cpp) into <documentDirectory>/models on first use,
   reporting 0..1 progress through the existing "Loading model… X%" UI.
   Downloads to a .part file and renames on success so an interrupted
   download can't leave a truncated model; if initWhisper still rejects,
   the file is deleted so the next try re-downloads cleanly. Model URLs
   verified (tiny.en 77.7MB / base.en 148MB / small.en 488MB).

2. Error surfacing. transcribeStore did `String(err)`, which renders a
   non-Error native rejection (whisper.rn / file system throw plain
   objects) as "[object Object]". New shared errorMessage() pulls
   message/reason/code (or JSON) out of whatever was thrown, so failures
   are actionable instead of opaque.

Progress downloads aren't in the new expo-file-system OO API yet, so the
fetch uses the still-supported expo-file-system/legacy resumable
downloader; native-only, not bundled on web.

Validated: tsc clean, 279 tests pass, web export unaffected (legacy not
in the web bundle), arm64 APK builds and the native JS bundle resolves
the legacy import. On-device download + transcribe still needs a real
phone to confirm end to end.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/lib/errorMessage.ts

@@ -0,0 +1,24 @@
+// Extract a human-readable message from an unknown thrown value.
+//
+// `String(err)` on a non-Error object yields the useless "[object Object]",
+// which is exactly what surfaced when native modules (whisper.rn, the audio
+// decoder, file system) reject with a plain `{ message, code }`-shaped object
+// instead of an `Error`. This pulls the most useful text out of whatever was
+// thrown so the UI shows something actionable.
+export function errorMessage(err: unknown): string {
+  if (err instanceof Error) return err.message;
+  if (typeof err === 'string') return err;
+  if (err && typeof err === 'object') {
+    const o = err as { message?: unknown; reason?: unknown; code?: unknown };
+    const parts = [o.message, o.reason, o.code]
+      .filter((x) => x != null && x !== '')
+      .map(String);
+    if (parts.length > 0) return parts.join(' — ');
+    try {
+      return JSON.stringify(err);
+    } catch {
+      /* fall through to String() for circular/unserializable objects */
+    }
+  }
+  return String(err);
+}