Make AI categorization prompts user-configurable

Add SystemPrompt and UserPromptTemplate fields to AI categorization
settings so users can customize the prompts sent to Ollama without
code changes. Null/empty values fall back to built-in defaults.

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

frontend/src/types/index.ts

@@ -493,6 +493,8 @@ export interface AiCategorizationSettings {
   confidenceThreshold: number
   timeoutSeconds: number
   updatePayeeDefaults: boolean
+  systemPrompt: string | null
+  userPromptTemplate: string | null
 }
 
 export interface OllamaConnectionTestResult {

frontend/src/views/settings/SettingsView.vue

@@ -466,6 +466,44 @@
           </v-card-text>
         </v-card>
 
+        <v-card class="mt-4">
+          <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.
+              The category list is always appended to the system prompt, and the transaction list is always appended to the user prompt.
+            </p>
+
+            <v-textarea
+              v-model="aiSettings.systemPrompt"
+              label="System Prompt"
+              variant="outlined"
+              rows="8"
+              :placeholder="defaultSystemPrompt"
+              persistent-placeholder
+              class="mb-4"
+            />
+
+            <v-textarea
+              v-model="aiSettings.userPromptTemplate"
+              label="User Prompt Template"
+              variant="outlined"
+              rows="3"
+              :placeholder="defaultUserPromptTemplate"
+              persistent-placeholder
+              class="mb-4"
+            />
+
+            <v-btn
+              variant="tonal"
+              color="secondary"
+              @click="resetPromptsToDefaults"
+            >
+              Reset to Defaults
+            </v-btn>
+          </v-card-text>
+        </v-card>
+
         <v-snackbar v-model="showAiSuccess" :timeout="4000" color="success">
           AI categorization settings saved
         </v-snackbar>
@@ -801,7 +839,9 @@ const aiSettings = reactive<AiCategorizationSettings>({
   modelName: 'llama3.1:8b',
   confidenceThreshold: 0.7,
   timeoutSeconds: 30,
-  updatePayeeDefaults: true
+  updatePayeeDefaults: true,
+  systemPrompt: null,
+  userPromptTemplate: null
 })
 const aiSaving = ref(false)
 const aiTesting = ref(false)
@@ -811,6 +851,31 @@ const showAiSuccess = ref(false)
 const showAiError = ref(false)
 const aiErrorMessage = ref('')
 
+const defaultSystemPrompt = `You are a personal finance transaction categorizer. Classify each transaction into the most appropriate category.
+
+You MUST respond with valid JSON in exactly this format:
+{
+  "classifications": [
+    {"index": 0, "categoryName": "Groceries", "confidence": 0.9},
+    {"index": 1, "categoryName": "Utilities", "confidence": 0.85}
+  ]
+}
+
+Rules:
+- "index" must be the transaction number (integer) from the list below
+- "categoryName" must be one of the exact category names listed below
+- "confidence" must be a number between 0 and 1
+- Include one entry for every transaction
+
+Available categories:`
+
+const defaultUserPromptTemplate = 'Classify these uncategorized transactions:'
+
+function resetPromptsToDefaults() {
+  aiSettings.systemPrompt = null
+  aiSettings.userPromptTemplate = null
+}
+
 async function loadAiSettings() {
   try {
     const { data } = await aiCategorizationApi.getSettings()