chore: Update README.md screenshot references and alt text (#8862)

docs: fix README screenshot references and clean up alt text

The Talk Interface was incorrectly using screenshot_tts.png (same as
Generate Audio) instead of screenshot_talk.png. Also standardized
alt text across all screenshot references for consistency.

Signed-off-by: localai-bot <localai-bot@noreply.github.com>
Co-authored-by: localai-bot <localai-bot@noreply.github.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>