feat: [US-060] - Edge condition variable dropdown

Replace the variableName text input in edge conditions with a
Combobox-based variable selector. Adds ConditionEditor modal
that opens on edge click, with type-aware operators (comparison
for numeric, == and != for string/boolean) and type-adaptive
value inputs (number, text, or boolean toggle).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/types/flowchart.ts

@@ -75,7 +75,7 @@ export type Condition = {
   variableName: string;
   variableId?: string;
   operator: '>' | '<' | '==' | '>=' | '<=' | '!=';
-  value: number;
+  value: number | string | boolean;
 };
 
 // FlowchartEdge type: represents connections between nodes