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-TW.ts

@@ -1416,6 +1416,12 @@ export const dict: Record<I18nKey, string> = {
   'chat.emptyState.startNewChat': '開始新的聊天',
   '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': '捲動到底部',
   'chat.timeline.relative.justNow': '剛剛',
   'chat.timeline.relative.minutesAgo': '{count} 分鐘前',
@@ -1476,6 +1482,7 @@ export const dict: Record<I18nKey, string> = {
   'chat.commandAutocomplete.command.compactDescription': '使用 AI 壓縮會話歷史以減少上下文大小',
   'chat.commandAutocomplete.command.summaryDescription': '非破壞性會話總結。命令後可選填主題提示。',
   'chat.commandAutocomplete.command.workspaceReviewDescription': '僅審查目前工作區中高價值的問題。',
+  'chat.commandAutocomplete.command.featurePlanDescription': '為新功能開始一次引導式的來回規劃工作階段。',
   'chat.commandAutocomplete.badge.skill': 'Skills',
   'chat.commandAutocomplete.badge.command': '命令',
   'chat.commandAutocomplete.badge.system': '系統',
@@ -1591,6 +1598,7 @@ export const dict: Record<I18nKey, string> = {
   'chat.chatInput.toast.compactFailed': '壓縮會話失敗',
   'chat.chatInput.toast.summaryFailed': '生成總結失敗',
   'chat.chatInput.toast.reviewFailed': '審查變更失敗',
+  'chat.chatInput.toast.planFeatureFailed': '無法開始功能規劃',
   'chat.chatInput.toast.attachmentsTooLarge': '附件過大，無法傳送。請減少圖片數量或大小。',
   'chat.chatInput.toast.sendAttachmentsFailed': '傳送附件失敗。請嘗試更少檔案或更小圖片。',
   'chat.chatInput.toast.messageSendFailed': '訊息傳送失敗，附件已恢復。',