Use editable default values with {placeholders} for AI prompts

Instead of showing defaults as greyed-out placeholder text, populate
textareas with the actual default prompt text so users can directly
edit it. Add {categories}, {examples}, and {transactions} placeholders
so users control where dynamic data is inserted in the prompts.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/views/settings/SettingsView.vue

@@ -470,8 +470,10 @@
           <v-card-title>Prompt Templates</v-card-title>
           <v-card-text>
             <p class="text-body-2 text-medium-emphasis mb-4">
-              Customize the prompts sent to the AI model. Leave empty to use built-in defaults.
+              Customize the prompts sent to the AI model. Use placeholders to control where dynamic data is inserted:
-              The category list is always appended to the system prompt, and the transaction list is always appended to the user prompt.
+              <strong>{categories}</strong> = category list,
+              <strong>{examples}</strong> = recent categorized transactions,
+              <strong>{transactions}</strong> = uncategorized transactions to classify.
             </p>
 
             <v-textarea
@@ -479,8 +481,6 @@
               label="System Prompt"
               variant="outlined"
               rows="8"
-              :placeholder="defaultSystemPrompt"
-              persistent-placeholder
               class="mb-4"
             />
 
@@ -488,9 +488,7 @@
               v-model="aiSettings.userPromptTemplate"
               label="User Prompt Template"
               variant="outlined"
-              rows="3"
+              rows="5"
-              :placeholder="defaultUserPromptTemplate"
-              persistent-placeholder
               class="mb-4"
             />
 
@@ -867,21 +865,27 @@ Rules:
 - "confidence" must be a number between 0 and 1
 - Include one entry for every transaction
 
-Available categories:`
+Available categories:
+{categories}`
 
-const defaultUserPromptTemplate = 'Classify these uncategorized transactions:'
+const defaultUserPromptTemplate = `{examples}Classify these uncategorized transactions:
+{transactions}`
 
 function resetPromptsToDefaults() {
-  aiSettings.systemPrompt = null
+  aiSettings.systemPrompt = defaultSystemPrompt
-  aiSettings.userPromptTemplate = null
+  aiSettings.userPromptTemplate = defaultUserPromptTemplate
 }
 
 async function loadAiSettings() {
   try {
     const { data } = await aiCategorizationApi.getSettings()
     Object.assign(aiSettings, data)
+    aiSettings.systemPrompt = aiSettings.systemPrompt || defaultSystemPrompt
+    aiSettings.userPromptTemplate = aiSettings.userPromptTemplate || defaultUserPromptTemplate
   } catch {
     // Not configured yet — defaults are fine
+    aiSettings.systemPrompt = defaultSystemPrompt
+    aiSettings.userPromptTemplate = defaultUserPromptTemplate
   }
 }
 

src/Purrse.Api/Services/AiCategorizationService.cs

@@ -35,9 +35,14 @@ public class AiCategorizationService : IAiCategorizationService
         - Include one entry for every transaction
 
         Available categories:
+        {categories}
         """;
 
-    internal const string DefaultUserPromptTemplate = "Classify these uncategorized transactions:";
+    internal const string DefaultUserPromptTemplate =
+        """
+        {examples}Classify these uncategorized transactions:
+        {transactions}
+        """;
 
     private static readonly JsonSerializerOptions JsonOptions = new()
     {
@@ -233,45 +238,64 @@ public class AiCategorizationService : IAiCategorizationService
 
     private static string BuildSystemMessage(AiCategorizationSettings settings, List<Category> categories)
     {
-        var sb = new StringBuilder();
+        var prompt = settings.SystemPrompt ?? DefaultSystemPrompt;
-        sb.AppendLine(settings.SystemPrompt ?? DefaultSystemPrompt);
 
+        var sb = new StringBuilder();
         foreach (var category in categories)
         {
             var parentInfo = category.Parent != null ? $" (under \"{category.Parent.Name}\")" : "";
             sb.AppendLine($"- \"{category.Name}\"{parentInfo}");
         }
+        var categoryList = sb.ToString();
 
-        return sb.ToString();
+        return prompt.Contains("{categories}")
+            ? prompt.Replace("{categories}", categoryList)
+            : prompt + "\n" + categoryList;
     }
 
     private static string BuildUserMessage(AiCategorizationSettings settings, List<Transaction> recentCategorized, List<Transaction> batch)
     {
-        var sb = new StringBuilder();
+        var prompt = settings.UserPromptTemplate ?? DefaultUserPromptTemplate;
 
+        // Build examples block
+        var examplesSb = new StringBuilder();
         if (recentCategorized.Count > 0)
         {
-            sb.AppendLine("Recent categorized transactions (for reference):");
+            examplesSb.AppendLine("Recent categorized transactions (for reference):");
             for (int i = 0; i < recentCategorized.Count; i++)
             {
                 var t = recentCategorized[i];
                 var categoryName = t.Category?.Parent != null
                     ? $"{t.Category.Parent.Name} > {t.Category.Name}"
                     : t.Category?.Name ?? "Unknown";
-                sb.AppendLine($"{i + 1}. \"{t.PayeeName}\" (${t.Amount:F2}) -> \"{categoryName}\"");
+                examplesSb.AppendLine($"{i + 1}. \"{t.PayeeName}\" (${t.Amount:F2}) -> \"{categoryName}\"");
             }
-            sb.AppendLine();
+            examplesSb.AppendLine();
         }
+        var examples = examplesSb.ToString();
 
-        sb.AppendLine(settings.UserPromptTemplate ?? DefaultUserPromptTemplate);
+        // Build transactions block
+        var txnSb = new StringBuilder();
         for (int i = 0; i < batch.Count; i++)
         {
             var t = batch[i];
             var type = t.Amount >= 0 ? "credit" : "debit";
-            sb.AppendLine($"{i}. \"{t.PayeeName}\" (${t.Amount:F2}, {type}, {t.Date:yyyy-MM-dd}, memo: \"{t.Memo}\")");
+            txnSb.AppendLine($"{i}. \"{t.PayeeName}\" (${t.Amount:F2}, {type}, {t.Date:yyyy-MM-dd}, memo: \"{t.Memo}\")");
         }
+        var transactions = txnSb.ToString();
 
-        return sb.ToString();
+        var result = prompt;
+        if (result.Contains("{examples}"))
+            result = result.Replace("{examples}", examples);
+        else
+            result = examples + result;
+
+        if (result.Contains("{transactions}"))
+            result = result.Replace("{transactions}", transactions);
+        else
+            result = result + "\n" + transactions;
+
+        return result;
     }
 
     private List<TransactionClassification> ParseClassifications(