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/input-box.tsx

@@ -60,6 +60,7 @@ import {
   DropdownMenuTrigger,
 } from "../ui/dropdown-menu";
 
+import { ModeHoverGuide } from "./mode-hover-guide";
 import { Tooltip } from "./tooltip";
 
 export function InputBox({
@@ -197,31 +198,42 @@ export function InputBox({
           </PromptInputActionMenu> */}
           <AddAttachmentsButton className="px-2!" />
           <PromptInputActionMenu>
-            <PromptInputActionMenuTrigger className="gap-1! px-2!">
-              <div>
-                {context.mode === "flash" && <ZapIcon className="size-3" />}
-                {context.mode === "thinking" && (
-                  <LightbulbIcon className="size-3" />
-                )}
-                {context.mode === "pro" && (
-                  <GraduationCapIcon className="size-3" />
-                )}
-                {context.mode === "ultra" && (
-                  <RocketIcon className="size-3 text-[#dabb5e]" />
-                )}
-              </div>
-              <div
-                className={cn(
-                  "text-xs font-normal",
-                  context.mode === "ultra" ? "golden-text" : "",
-                )}
-              >
-                {(context.mode === "flash" && t.inputBox.flashMode) ||
-                  (context.mode === "thinking" && t.inputBox.reasoningMode) ||
-                  (context.mode === "pro" && t.inputBox.proMode) ||
-                  (context.mode === "ultra" && t.inputBox.ultraMode)}
-              </div>
-            </PromptInputActionMenuTrigger>
+            <ModeHoverGuide
+              mode={
+                context.mode === "flash" ||
+                context.mode === "thinking" ||
+                context.mode === "pro" ||
+                context.mode === "ultra"
+                  ? context.mode
+                  : "flash"
+              }
+            >
+              <PromptInputActionMenuTrigger className="gap-1! px-2!">
+                <div>
+                  {context.mode === "flash" && <ZapIcon className="size-3" />}
+                  {context.mode === "thinking" && (
+                    <LightbulbIcon className="size-3" />
+                  )}
+                  {context.mode === "pro" && (
+                    <GraduationCapIcon className="size-3" />
+                  )}
+                  {context.mode === "ultra" && (
+                    <RocketIcon className="size-3 text-[#dabb5e]" />
+                  )}
+                </div>
+                <div
+                  className={cn(
+                    "text-xs font-normal",
+                    context.mode === "ultra" ? "golden-text" : "",
+                  )}
+                >
+                  {(context.mode === "flash" && t.inputBox.flashMode) ||
+                    (context.mode === "thinking" && t.inputBox.reasoningMode) ||
+                    (context.mode === "pro" && t.inputBox.proMode) ||
+                    (context.mode === "ultra" && t.inputBox.ultraMode)}
+                </div>
+              </PromptInputActionMenuTrigger>
+            </ModeHoverGuide>
             <PromptInputActionMenuContent className="w-80">
               <DropdownMenuGroup>
                 <DropdownMenuLabel className="text-muted-foreground text-xs">