Persist text elements through TUI input and history

[charley-oai]

Summary

• Thread user text element ranges through TUI/TUI2 input, submission, queueing, and history so placeholders survive resume/edit flows.
• Preserve local image attachments alongside text elements and rehydrate placeholders when restoring drafts.
• Keep model-facing content shapes clean by attaching UI metadata only to user input/events (no API content changes).

Key Changes

• TUI/TUI2 composer now captures text element ranges, trims them with text edits, and restores them when submission is suppressed.
• User history cells render styled spans for text elements and keep local image paths for future rehydration.
• Initial chat widget bootstraps accept empty initial_text_elements to keep initialization uniform.
• Protocol/core helpers updated to tolerate the new InputText field shape without changing payloads sent to the API.