Add reset-to-default button and how-to docs to system prompt editor

Each service prompt card now shows:
- A collapsible how-to panel with placeholder docs, required JSON
  response keys, and usage notes
- A "Reset to Default" button (with confirmation step) that restores
  the built-in prompt without saving, letting the admin review first
- A "Using the built-in default prompt" indicator when unchanged

Backend includes default_system / default_user_template in the
system-prompts API response so the frontend never duplicates defaults.

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

frontend/src/api/client.ts

@@ -225,6 +225,8 @@ export interface ServiceSystemPrompt {
   label: string;
   system: string;
   user_template: string;
+  default_system: string;
+  default_user_template: string;
 }
 
 export type SystemPromptsData = Record<string, ServiceSystemPrompt>;