refactor: improve code quality based on Gemini Code Assist suggestions

Applied three code quality improvements suggested by Gemini Code Assist:

1. **Replace nested ternary with map object (enhance.ts)**
   - Changed nested ternary operator to Record<EnhancementMode, string> map
   - Improves readability and maintainability
   - More declarative approach for system prompt selection

2. **Simplify handleToggle logic (prompt-customization-section.tsx)**
   - Removed redundant if/else branches
   - Both branches were calculating the same value
   - Cleaner, more concise implementation

3. **Add type safety to updatePrompt with generics (prompt-customization-section.tsx)**
   - Changed field parameter from string to keyof NonNullable<PromptCustomization[T]>
   - Prevents runtime errors from misspelled field names
   - Improved developer experience with autocomplete

All tests passing (774/774). Builds successful.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

apps/server/src/routes/enhance-prompt/routes/enhance.ts

@@ -136,14 +136,13 @@ export function createEnhanceHandler(
       const prompts = await getPromptCustomization(settingsService, '[EnhancePrompt]');
 
       // Get the system prompt for this mode from merged prompts
-      const systemPrompt =
-        validMode === 'improve'
-          ? prompts.enhancement.improveSystemPrompt
-          : validMode === 'technical'
-            ? prompts.enhancement.technicalSystemPrompt
-            : validMode === 'simplify'
-              ? prompts.enhancement.simplifySystemPrompt
-              : prompts.enhancement.acceptanceSystemPrompt;
+      const systemPromptMap: Record<EnhancementMode, string> = {
+        improve: prompts.enhancement.improveSystemPrompt,
+        technical: prompts.enhancement.technicalSystemPrompt,
+        simplify: prompts.enhancement.simplifySystemPrompt,
+        acceptance: prompts.enhancement.acceptanceSystemPrompt,
+      };
+      const systemPrompt = systemPromptMap[validMode];
 
       logger.debug(`Using ${validMode} system prompt (length: ${systemPrompt.length} chars)`);