feat: user-customizable draft welcome starters

Let users curate the draft welcome chips: pin existing commands and skills
as starters, remove them, and drag to reorder — all inline on the draft
screen via a '+' picker dialog and per-chip remove, with no separate
settings UI.

A starter references a command or skill; its scope is inherited from the
item (user-scope -> global, project-scope -> per-project). Global starters
persist to settings.json (useUIStore + client/server sanitizers); project
starters persist to the project config alongside worktree setup commands.
The two scopes form ordered namespaces shown global-first then project,
reorderable only within each group.

The six built-in Session magic-prompt commands are the default global set
and stay available in the picker for re-pinning if removed; they keep their
bespoke icons, while user commands/skills fall back to the Commands/Skills
section icons. Chip labels are normalized (/simplify-code -> 'Simplify
code'). Missing commands/skills are skipped rather than shown broken.

Drag-to-reorder works on desktop and mobile: rectSortingStrategy for the
wrapping multi-row layout, CSS.Translate (no scale) so the lifted chip
doesn't stretch, and MouseSensor + long-press TouchSensor so taps still
submit and swipes still scroll. The '+' picker is a searchable dialog on
every surface.

packages/ui/src/lib/i18n/messages/ko.ts

@@ -1461,6 +1461,13 @@ export const dict: Record<I18nKey, string> = {
   'chat.draftPresets.plan.label': 'Start feature planning',
   'chat.draftPresets.debug.label': 'Debug an issue',
   'chat.draftPresets.review.label': 'Review my changes',
+  'chat.draftStarters.add': 'Add a starter',
+  'chat.draftStarters.searchPlaceholder': 'Search commands and skills…',
+  'chat.draftStarters.empty': 'Nothing to add',
+  'chat.draftStarters.sectionBuiltIn': 'Built-in',
+  'chat.draftStarters.sectionCommands': 'Commands',
+  'chat.draftStarters.sectionSkills': 'Skills',
+  'chat.draftStarters.remove': 'Remove',
   'chat.scrollToBottom.aria': '맨 아래로 스크롤',
   'chat.timeline.relative.justNow': '방금 전',
   'chat.timeline.relative.minutesAgo': '{count}분 전',