fix(viewer): byte-faithful saves, poll/save races, clearer errors

- Keep CRLF (or CR) line endings and a UTF-8 BOM through the editor:
  textFormat.ts records the dominant separator and the BOM on load and
  restores both on save, so a save changes only the user's edits.
- A clean document whose reload failed retries on the next poll.
- A poll that overlaps a save, or was issued before one settled, is
  ignored instead of reading the pre-save hash as a change.
- A conflict whose follow-up poll has no hash shows an error with a
  Reload button rather than an Overwrite that could only conflict again.
- Match write.rs's exact read-only message; show the read-only reason as
  visible text; error banners are role="alert".
- vite/client types move to src/vite-env.d.ts.
- Tests: CRLF and BOM saves, reload retry, poll/save race, null-hash
  conflict, Save and close success and failure, and CodeEditor.setDoc
  keeping cursor and scroll.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

app/src/vite-env.d.ts

@@ -0,0 +1 @@
+/// <reference types="vite/client" />