Record the product audience and copy tone in PRODUCT.md

Primary users are non-technical people who want Whisper without docs or
ML setup; technical users must keep full control as if running it on
their own device.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

PRODUCT.md

@@ -4,6 +4,23 @@ Whisper Web — on-device speech-to-text. A browser-first transcription app powe
 running through [Transformers.js](https://github.com/huggingface/transformers) (`@huggingface/transformers`).
 (Moved here from the original `docs/CLAUDE.md` §1.)
 
+## Audience
+
+**Primary: non-technical people** who want Whisper transcription without reading docs or setting up
+any ML tooling themselves. **Also: technical people**, who must be able to do what they want and need
+with it — the same control they'd have running Whisper on their own device.
+
+What that means for the UI and its copy:
+- **Works with zero knowledge.** Defaults are good enough to just drop a file or press record and get a
+  transcript. No step should require knowing what a backend, dtype, or quantization is.
+- **Plain language first.** Labels, statuses, and errors say what is happening and what to do next in
+  everyday words ("Downloading the model (~44 MB, one time)", not "Fetching ONNX weights").
+- **Control is there, not in the way.** Technical choices (model size/variant, backend, language, task,
+  timestamps, export formats, storage) stay available and honest, but visually secondary — never
+  hidden behind a "simple mode" that removes them, and never required.
+- **Technical truth on demand.** Where a technical term is the real answer (WebGPU, WASM, model ids),
+  show it alongside the plain description rather than replacing it.
+
 ## Users and what they get
 
 **Anonymous users (no login) can:**
@@ -36,5 +53,3 @@ running through [Transformers.js](https://github.com/huggingface/transformers) (
 
 Arbitrary Hub model ids, OAuth providers, multi-user transcript storage, whisper.cpp server backend,
 i18n of the UI.
-
-TODO(maintainer): confirm target audience and tone of voice for UI copy — not recorded anywhere.