feat: desktop draft welcome presets and /plan-feature command

Add starter preset chips under the composer on the desktop draft welcome
screen (explore, what changed, plan, review). Clicking a chip submits
immediately; prompt chips send a natural-language prompt, command chips
reuse built-in slash commands.

Add a new /plan-feature built-in command, modeled on /workspace-review:
visible + hidden magic prompts that run a guided, batched-question
planning dialogue (investigate the code, ask up to 3 clarifying
questions at a time, surface pitfalls, then produce an implementation
plan). Wired into command autocomplete, the submit handler, the draft
plan chip, and the Magic Prompts settings page, with i18n across all
locales.

packages/ui/src/lib/i18n/messages/zh-CN.ts

@@ -1419,6 +1419,12 @@ export const dict: Record<I18nKey, string> = {
   'chat.emptyState.startNewChat': '开始新的聊天',
   'chat.emptyState.draftTitle': 'What are we working on?',
   'chat.emptyState.draftTitleWithProject': 'What are we working on in {project}?',
+  'chat.draftPresets.explore.label': 'Explore the codebase',
+  'chat.draftPresets.explore.prompt': 'Give me a high-level tour of this codebase — the architecture, the main modules, and how they fit together.',
+  'chat.draftPresets.changes.label': 'What changed recently',
+  'chat.draftPresets.changes.prompt': 'Summarize what changed recently — the latest commits and the current state of this branch.',
+  'chat.draftPresets.plan.label': 'Start feature planning',
+  'chat.draftPresets.review.label': 'Review my changes',
   'chat.scrollToBottom.aria': '滚动到底部',
   'chat.timeline.relative.justNow': '刚刚',
   'chat.timeline.relative.minutesAgo': '{count} 分钟前',
@@ -1479,6 +1485,7 @@ export const dict: Record<I18nKey, string> = {
   'chat.commandAutocomplete.command.compactDescription': '使用 AI 压缩会话历史以减少上下文大小',
   'chat.commandAutocomplete.command.summaryDescription': '非破坏性会话总结。命令后可选填主题提示。',
   'chat.commandAutocomplete.command.workspaceReviewDescription': '仅审查当前工作区中高价值的问题。',
+  'chat.commandAutocomplete.command.featurePlanDescription': '为新功能开始一次引导式的来回规划会话。',
   'chat.commandAutocomplete.badge.skill': '技能',
   'chat.commandAutocomplete.badge.command': '命令',
   'chat.commandAutocomplete.badge.system': '系统',
@@ -1594,6 +1601,7 @@ export const dict: Record<I18nKey, string> = {
   'chat.chatInput.toast.compactFailed': '压缩会话失败',
   'chat.chatInput.toast.summaryFailed': '生成总结失败',
   'chat.chatInput.toast.reviewFailed': '审查变更失败',
+  'chat.chatInput.toast.planFeatureFailed': '无法开始功能规划',
   'chat.chatInput.toast.attachmentsTooLarge': '附件过大，无法发送。请减少图片数量或大小。',
   'chat.chatInput.toast.sendAttachmentsFailed': '发送附件失败。请尝试更少文件或更小图片。',
   'chat.chatInput.toast.messageSendFailed': '消息发送失败，附件已恢复。',