feat: add /weigh approach-comparison command to draft welcome

Add /weigh as a draft chip and command for the moment before planning —
when you know what to build but not how. Its hidden prompt investigates
the code, then lays out 2-3 genuinely distinct approaches with trade-offs
(complexity, risk, blast radius, effort) and a clear recommendation,
without writing a plan or code.

Follows the established command pattern: visible + hidden magic prompts
wired into command autocomplete, the submit handler, draft preset chips,
and the Magic Prompts settings page, with i18n across all locales. Adds a
scales-3 icon to the sprite.

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

@@ -1419,6 +1419,7 @@ export const dict: Record<I18nKey, string> = {
   '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.catchup.label': 'Catch me up',
+  'chat.draftPresets.weigh.label': 'Weigh my options',
   'chat.draftPresets.plan.label': 'Start feature planning',
   'chat.draftPresets.debug.label': 'Debug an issue',
   'chat.draftPresets.review.label': 'Review my changes',
@@ -1485,6 +1486,7 @@ export const dict: Record<I18nKey, string> = {
   '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.command.catchUpDescription': '重新進入上下文：你之前在做什麼、從哪裡繼續。',
   'chat.commandAutocomplete.command.debugDescription': '在提出修復方案前，引導式地排查 bug 的根本原因。',
+  'chat.commandAutocomplete.command.weighDescription': '在動手前，權衡 2-3 種方案的利弊並給出推薦。',
   'chat.commandAutocomplete.badge.skill': 'Skills',
   'chat.commandAutocomplete.badge.command': '命令',
   'chat.commandAutocomplete.badge.system': '系統',
@@ -1603,6 +1605,7 @@ export const dict: Record<I18nKey, string> = {
   'chat.chatInput.toast.planFeatureFailed': '無法開始功能規劃',
   'chat.chatInput.toast.catchUpFailed': '無法取得上下文',
   'chat.chatInput.toast.debugFailed': '無法開始除錯',
+  'chat.chatInput.toast.weighFailed': '無法權衡方案',
   '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。',