Fix: collapsible thinking display and separate deep research retrieval tag (#14613)

## Summary

- **Collapsible thinking**: Replace `<section>` with `<details>` for
`<think>` content, so model thinking output is collapsed by default
(click to expand). Works for all models that output `<think>` tags
(Qwen3, DeepSeek, Gemini, Claude, etc.).
- **Fix double thinking tags**: When reasoning/deep research mode is
enabled in knowledge base chat, both the retrieval progress and model
thinking were wrapped in `<think>` tags, producing two "Thinking..."
blocks. Now retrieval progress uses a dedicated `<retrieving>` tag
rendered as a separate "Retrieving..." collapsible with a distinct green
accent.

### Before
- Thinking content displayed as flat gray-bordered `<section>`,
occupying significant screen space
- Deep research + model thinking both use `<think>` → two identical
"Thinking..." blocks

### After
- Thinking content collapsed by default in a `<details>` element, click
"Thinking..." to expand
- Deep research shows "Retrieving..." (green border), model thinking
shows "Thinking..." (gray border)

## Changes

**Backend (`api/db/services/dialog_service.py`)**
- Deep research callback: replace `start_to_think`/`end_to_think` marker
flags with direct `<retrieving>`/`</retrieving>` answer text

**Frontend**
- `web/src/utils/chat.ts`: `replaceThinkToSection()` now uses
`<details>` instead of `<section>`; add new
`replaceRetrievingToSection()`
- 4 tsx files: import and pipe `replaceRetrievingToSection`, whitelist
`details`, `summary`, `retrieving` in DOMPurify `ADD_TAGS`
- 4 less files: `section.think` → `details.think` with `<summary>`
styles; add `details.retrieving` with green accent; dark mode and RTL
variants

## Test plan
- [ ] Open a chat WITHOUT knowledge base, ask a question to a model with
thinking (e.g. Qwen3) → thinking content should be collapsed by default,
click "Thinking..." to expand
- [ ] Open a chat WITH knowledge base and reasoning enabled, ask a
question → "Retrieving..." (green) shows retrieval progress,
"Thinking..." (gray) shows model thinking, each independently
collapsible
- [ ] Verify dark mode renders correctly for both collapsible blocks
- [ ] Verify RTL layout renders correctly

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: wanghualoong <wanghualoong@gmail.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

web/src/utils/chat.ts

@@ -81,7 +81,15 @@ export const preprocessLaTeX = (content: string) => {
 export function replaceThinkToSection(text: string = '') {
   const pattern = /<think>([\s\S]*?)<\/think>/g;
 
-  const result = text.replace(pattern, '<section class="think">$1</section>');
+  const result = text.replace(pattern, '<details class="think"><summary>Thinking...</summary>$1</details>');
+
+  return result;
+}
+
+export function replaceRetrievingToSection(text: string = '') {
+  const pattern = /<retrieving>([\s\S]*?)<\/retrieving>/g;
+
+  const result = text.replace(pattern, '<details class="retrieving"><summary>Retrieving...</summary>$1</details>');
 
   return result;
 }