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/components/sections/magic-prompts/MagicPromptsPage.tsx

@@ -140,6 +140,14 @@ const PROMPT_PAGE_MAP: Record<string, PromptPageConfig> = {
       { id: 'session.review.instructions', titleKey: 'settings.magicPrompts.page.block.instructions' },
     ],
   },
+  'session.plan': {
+    titleKey: 'settings.magicPrompts.page.group.sessionFeaturePlan.title',
+    descriptionKey: 'settings.magicPrompts.page.group.sessionFeaturePlan.description',
+    blocks: [
+      { id: 'session.plan.visible', titleKey: 'settings.magicPrompts.page.block.visiblePrompt' },
+      { id: 'session.plan.instructions', titleKey: 'settings.magicPrompts.page.block.instructions' },
+    ],
+  },
   'session.fusion': {
     titleKey: 'settings.magicPrompts.page.group.sessionFusion.title',
     descriptionKey: 'settings.magicPrompts.page.group.sessionFusion.description',

packages/ui/src/components/sections/magic-prompts/MagicPromptsSidebar.tsx

@@ -47,6 +47,7 @@ export const MagicPromptsSidebar: React.FC<MagicPromptsSidebarProps> = ({ onItem
         items: [
           { id: 'session.summary', titleKey: 'settings.magicPrompts.sidebar.item.sessionSummary' },
           { id: 'session.review', titleKey: 'settings.magicPrompts.sidebar.item.sessionWorkspaceReview' },
+          { id: 'session.plan', titleKey: 'settings.magicPrompts.sidebar.item.sessionFeaturePlan' },
           { id: 'session.fusion', titleKey: 'settings.magicPrompts.sidebar.item.sessionFusion' },
         ],
       },