feat: add reasoning content to AI conversation records and update related components

[hgaol]

Fix #1524

Root cause

DeepSeek's reasoning models stream reasoning_content alongside content. Answer ignored it, so follow-up requests failed with 400: The reasoning_content in the thinking mode must be passed back to the API, and the thinking text was never shown or saved.

Fix

• Capture reasoning_content from the stream and pass it back to theAPI on subsequent rounds.
• Persist it with the conversation (new DB column via migrationv2.0.2).
• Render it in the chat UI as a collapsible "Thinking…/Thoughts"panel above the answer.

Compatibility

Nullable column, omitempty field, UI hides the panel when empty — old conversations and non-reasoning models behave exactly as before.

Demo

Screen.Recording.2026-05-21.at.22.28.56.mov

[LinkinStars]

@hgaol This branch has conflicts that must be resolved. Once the conflicts are resolved, we will merge this PR.