feat(S2): max_tokens dynamic from template, remove hardcoded 500

- PromptTemplate model: add max_tokens (default 500) and example_input fields
- generate_with_llm: read max_tokens from template, accept override param
- api_prompt_generate: accept optional max_tokens with 100-4096 clamp
- Vue: add advanced options toggle with max_tokens input
- Vue: auto-populate max_tokens from selected template

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

vue-app/src/api/modules/prompt.ts

@@ -14,6 +14,6 @@ export function fetchTemplatesByCategory(category: string) {
   return client.get<TemplatesByCategoryResponse>(path).then((r) => r.data)
 }
 
-export function generatePrompt(body: { input_text: string; template_id: number | null }) {
+export function generatePrompt(body: { input_text: string; template_id: number | null; max_tokens?: number }) {
   return client.post<GeneratePromptResponse>('/api/prompt/generate', body).then((r) => r.data)
 }