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/components/CreateThreadModal.tsx

@@ -112,7 +112,7 @@ export function CreateThreadModal() {
 						>
 							{categories?.map((cat) => (
 								<option key={cat.id} value={cat.id}>
-									{cat.emoji} {cat.name}
+									{cat.name}
 								</option>
 							))}
 						</select>