feat: AI Copilot suggestion chips + typeahead dropdown

- Suggestion chips bar: context-aware (amber for rearrange, indigo for creation)
- Typeahead dropdown: keyword matching on ≥2 chars, Tab to select
- getSelectedMidiItemInfo helper for context detection
- Backend: 4 rearrange presets in ai_presets.py
- PromptTemplateManager: getContextualSuggestions(hasSelectedItem)

app/static/js/services/promptTemplateManager.js

@@ -236,6 +236,14 @@ const PromptTemplateManager = (function() {
     this.savePresets();
   };
 
+  PromptTemplateManager.prototype.getContextualSuggestions = function(hasSelectedItem) {
+    if (hasSelectedItem) {
+      return this.presets.filter(p => p.category === 'Rearrange / Variation' || p.is_favorite);
+    } else {
+      return this.presets.filter(p => p.category !== 'Rearrange / Variation');
+    }
+  };
+
   PromptTemplateManager.prototype.toggleFavorite = function(id) {
     const preset = this.presets.find(p => p.id === id);
     if (preset) {