iOS: composer does not auto-capitalize first letter of new sentence

**Describe the bug**

On iOS, the message composer does not auto-capitalize the first letter of a new sentence. The system keyboard stays in lowercase even after a period + space, so users have to manually toggle shift to start each sentence with a capital letter.

This happens in every message field (DMs and channel composers). Other iOS apps (Messages, Mail, Notes) auto-capitalize as expected.

**Steps to reproduce**

1. Open Buzz on iPhone
2. Start typing in the message composer: "hello there. "
3. After typing the period and a space, the next letter is still lowercase: "hello there. how"
4. Expected: "hello there. How"

**Expected behavior**

The keyboard's autocapitalization should follow iOS defaults for sentence input: capitalize the first letter of each sentence (and the very first word of the message). In Flutter terms, the composer `TextField`s should use `textCapitalization: TextCapitalization.sentences` (the Flutter default) rather than `none`.

**Version and platform**

- Buzz version: iOS App Store build (latest, September 2026)
- OS: iOS (iPhone)

**Logs / additional context**

None found searching the issue tracker for "autocapitalize", "capital letter", "textCapitalization" and similar terms — this appears unreported.