feat(frontend): add mode hover guide and adjust mode i18n

## 中文

### 代码改动
- **新增** `frontend/src/components/workspace/mode-hover-guide.tsx`
  - 新增 ModeHoverGuide 组件：接收 mode (flash/thinking/pro/ultra) 与 children，用 Tooltip 包裹
  - hover 时展示该模式名称与简介，支持 showTitle 控制是否显示模式名
  - 文案通过 useI18n 从 inputBox 的 *Mode / *ModeDescription 读取，中英文已支持
- **修改** `frontend/src/components/workspace/input-box.tsx`
  - 在模式选择器触发按钮外包一层 ModeHoverGuide，悬停当前模式时显示说明
- **修改** `frontend/src/core/i18n/locales/zh-CN.ts`
  - ultraModeDescription：改为完整描述「思考、计划并执行，可调用子代理分工协作，适合复杂多步骤任务，能力最强」（不再仅写「专业模式加子代理」）
  - proMode / ultraMode：中文环境下保留英文原文 "Pro"、"Ultra"，不再翻译为「专业」「超级」
- **修改** `frontend/src/core/i18n/locales/en-US.ts`
  - ultraModeDescription：改为 "Reasoning, planning and execution with subagents to divide work; best for complex multi-step tasks"

### 说明
为 Flash / 思考 / Pro / Ultra 四种模式增加 hover 说明，并统一超级模式文案与 Pro/Ultra 在中文下的展示。

Co-authored-by: Cursor <cursoragent@cursor.com>

---

## English

### Code changes
- **Add** `frontend/src/components/workspace/mode-hover-guide.tsx`
  - New ModeHoverGuide component: takes mode (flash/thinking/pro/ultra) and children, wraps in Tooltip
  - On hover shows mode name and short description; showTitle toggles mode name in tooltip
  - Copy from useI18n (inputBox *Mode / *ModeDescription), i18n in zh-CN and en-US
- **Update** `frontend/src/components/workspace/input-box.tsx`
  - Wrap mode selector trigger with ModeHoverGuide so hovering shows current mode description
- **Update** `frontend/src/core/i18n/locales/zh-CN.ts`
  - ultraModeDescription: full description (reasoning, planning, execution, subagents, complex tasks); no longer "Pro + subagents" only
  - proMode / ultraMode: keep English "Pro" and "Ultra" in zh locale instead of "专业" / "超级"
- **Update** `frontend/src/core/i18n/locales/en-US.ts`
  - ultraModeDescription: "Reasoning, planning and execution with subagents to divide work; best for complex multi-step tasks"

### Summary
Hover guide for all four modes (Flash / Reasoning / Pro / Ultra); clearer Ultra copy and Pro/Ultra labels in Chinese.

frontend/src/core/i18n/locales/en-US.ts

@@ -81,7 +81,7 @@ export const enUS: Translations = {
       "Reasoning, planning and executing, get more accurate results, may take more time",
     ultraMode: "Ultra",
     ultraModeDescription:
-      "Pro mode with subagents enabled, maximum capability for complex tasks",
+      "Reasoning, planning and execution with subagents to divide work; best for complex multi-step tasks",
     searchModels: "Search models...",
     surpriseMe: "Surprise",
     surpriseMePrompt: "Surprise me",

frontend/src/core/i18n/locales/zh-CN.ts

@@ -75,11 +75,11 @@ export const zhCN: Translations = {
     flashModeDescription: "快速且高效的完成任务，但可能不够精准",
     reasoningMode: "思考",
     reasoningModeDescription: "思考后再行动，在时间与准确性之间取得平衡",
-    proMode: "专业",
+    proMode: "Pro",
     proModeDescription: "思考、计划再执行，获得更精准的结果，可能需要更多时间",
-    ultraMode: "超级",
+    ultraMode: "Ultra",
     ultraModeDescription:
-      "专业模式加子代理，适用于复杂的多步骤任务，功能最强大",
+      "思考、计划并执行，可调用子代理分工协作，适合复杂多步骤任务，能力最强",
     searchModels: "搜索模型...",
     surpriseMe: "小惊喜",
     surpriseMePrompt: "给我一个小惊喜吧",