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/en.settings.ts

@@ -223,6 +223,7 @@ export const settingsDict = {
   'settings.magicPrompts.sidebar.item.planImplement': 'Implement Plan',
   'settings.magicPrompts.sidebar.item.sessionSummary': 'Session Summary',
   'settings.magicPrompts.sidebar.item.sessionWorkspaceReview': 'Workspace Review',
+  'settings.magicPrompts.sidebar.item.sessionFeaturePlan': 'Feature Planning',
   'settings.magicPrompts.sidebar.item.sessionFusion': 'Fusion',
   'settings.remoteInstances.sidebar.title': 'Remote Instances',
   'settings.remoteInstances.sidebar.total': 'Total {count}',
@@ -1686,6 +1687,8 @@ export const settingsDict = {
   'settings.magicPrompts.page.group.sessionSummary.description': 'Prompts used by the /summary slash command: visible user message + hidden instructions. Non-destructive - does not compact session history.',
   'settings.magicPrompts.page.group.sessionWorkspaceReview.title': 'Workspace Review',
   'settings.magicPrompts.page.group.sessionWorkspaceReview.description': 'Prompts used by the /workspace-review slash command: visible user message + hidden instructions. Reviews current workspace changes for high-signal issues only.',
+  'settings.magicPrompts.page.group.sessionFeaturePlan.title': 'Feature Planning',
+  'settings.magicPrompts.page.group.sessionFeaturePlan.description': 'Prompts used by the /plan-feature slash command: visible user message + hidden instructions. Runs a guided dialogue that researches the code and asks clarifying questions in small batches before producing an implementation plan.',
   'settings.magicPrompts.page.group.sessionFusion.title': 'Fusion',
   'settings.magicPrompts.page.group.sessionFusion.description': 'Prompts used when combining multi-run outputs into one final answer: visible user message + hidden instructions before source results.',
   'settings.magicPrompts.page.actions.resetting': 'Resetting...',