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/components/workspace/mode-hover-guide.tsx

@@ -0,0 +1,60 @@
+"use client";
+
+import { useI18n } from "@/core/i18n/hooks";
+import { Tooltip } from "./tooltip";
+
+export type AgentMode = "flash" | "thinking" | "pro" | "ultra";
+
+function getModeLabelKey(
+  mode: AgentMode,
+): keyof Pick<
+  import("@/core/i18n/locales/types").Translations["inputBox"],
+  "flashMode" | "reasoningMode" | "proMode" | "ultraMode"
+> {
+  switch (mode) {
+    case "flash":
+      return "flashMode";
+    case "thinking":
+      return "reasoningMode";
+    case "pro":
+      return "proMode";
+    case "ultra":
+      return "ultraMode";
+  }
+}
+
+function getModeDescriptionKey(
+  mode: AgentMode,
+): keyof Pick<
+  import("@/core/i18n/locales/types").Translations["inputBox"],
+  "flashModeDescription" | "reasoningModeDescription" | "proModeDescription" | "ultraModeDescription"
+> {
+  switch (mode) {
+    case "flash":
+      return "flashModeDescription";
+    case "thinking":
+      return "reasoningModeDescription";
+    case "pro":
+      return "proModeDescription";
+    case "ultra":
+      return "ultraModeDescription";
+  }
+}
+
+export function ModeHoverGuide({
+  mode,
+  children,
+  showTitle = true,
+}: {
+  mode: AgentMode;
+  children: React.ReactNode;
+  /** When true, tooltip shows "ModeName: Description". When false, only description. */
+  showTitle?: boolean;
+}) {
+  const { t } = useI18n();
+  const label = t.inputBox[getModeLabelKey(mode)];
+  const description = t.inputBox[getModeDescriptionKey(mode)];
+  const content = showTitle ? `${label}: ${description}` : description;
+
+  return <Tooltip content={content}>{children}</Tooltip>;
+}