Bug fix: reliably detect UserMessage boundaries via typed spans

[warpdev]

Fixes #2755
Replaces #2757 to fix root cause.

Summary

Replace fragile string-based detection of user messages with typed metadata and span indices for backtracking. Only User messages are typed in this PR; others remain Other.

Root cause

Backtrack relied on rendered text parsing (“user” header join/trim), which loses type info and misidentifies boundaries.

Fix

• Add MessageKind and MessageSpan to HistoryCell (defaults keep existing cells unchanged).
• Introduce UserHistoryCell and return it from new_user_prompt (rendering unchanged).
• Track absolute user spans in App (user_spans: Vec<(header_abs, end_abs)>). Other spans is not necessary now.
• Refactor backtrack helpers to use spans (no string parsing).

Before

2025-08-29.2.07.37.mov

After

2025-08-29.2.06.51.mov

[warpdev]

Update: Here’s the result video after the TUI rework(#2877).

Before

2025-09-03.12.40.25.mov

After

2025-09-03.12.39.21.mov

[warpdev]

Hi — This PR fixes a bug where Resume may select the wrong user message. When you have a moment, could you please take a look so others don’t run into it? Thank you! @nornagon-openai @easong-openai @bolinfest