refactor: drop dead code left over from Cloudflare era

- Remove vietnamese-number.js + test: build-time Python script
  (generate-audio.py) is the sole spec for pre-baked MP3s; runtime
  no longer reads the JS module.
- Remove resetMaster() export: only ever called from tests; inline
  the two-line body at test sites.
- Update Python docstring to drop the JS-mirror reference.
- Rewrite stale "CF Pages root" comment in svelte.config.js.

web/scripts/generate-audio.py

@@ -23,7 +23,7 @@ ONES = [
 
 
 def number_to_vietnamese(n: int) -> str:
-    """Mirror of src/lib/vietnamese-number.js for build-time use."""
+    """Canonical Vietnamese number speller for build-time audio generation."""
     if n < 10:
         return ONES[n]
     if n == 10: