feat(06-02): replace EmojiPicker with IconPicker across all category components

- CategoryPicker shows LucideIcon prefix and uses IconPicker for inline create
- CategoryHeader displays LucideIcon in view mode and IconPicker in edit mode
- OnboardingWizard uses IconPicker for category creation step
- CreateThreadModal drops emoji from category select options
- Fixed categoryEmoji -> categoryIcon in routes and useCategories hook

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/client/routes/threads/$threadId.tsx

@@ -134,7 +134,7 @@ function ThreadDetailPage() {
               weightGrams={candidate.weightGrams}
               priceCents={candidate.priceCents}
               categoryName={candidate.categoryName}
-              categoryEmoji={candidate.categoryEmoji}
+              categoryIcon={candidate.categoryIcon}
               imageFilename={candidate.imageFilename}
               threadId={threadId}
               isActive={isActive}