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-CN.ts

@@ -1422,6 +1422,7 @@ 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',
@@ -1488,6 +1489,7 @@ export const dict: Record<I18nKey, string> = {
   'chat.commandAutocomplete.command.featurePlanDescription': '为新功能开始一次引导式的来回规划会话。',
   'chat.commandAutocomplete.command.catchUpDescription': '重新进入上下文：你之前在做什么、从哪里继续。',
   'chat.commandAutocomplete.command.debugDescription': '在提出修复方案前，引导式地排查 bug 的根本原因。',
+  'chat.commandAutocomplete.command.weighDescription': '在动手前，权衡 2-3 种方案的利弊并给出推荐。',
   'chat.commandAutocomplete.badge.skill': '技能',
   'chat.commandAutocomplete.badge.command': '命令',
   'chat.commandAutocomplete.badge.system': '系统',
@@ -1606,6 +1608,7 @@ export const dict: Record<I18nKey, string> = {
   'chat.chatInput.toast.planFeatureFailed': '无法开始功能规划',
   'chat.chatInput.toast.catchUpFailed': '无法获取上下文',
   'chat.chatInput.toast.debugFailed': '无法开始调试',
+  'chat.chatInput.toast.weighFailed': '无法权衡方案',
   '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。',