Add aiSystemPrompt to AppConfig (interface, Zod schema, defaults)
for users to override the base system prompt template.
buildSystemPrompt() accepts optional customPrompt param. When set,
it replaces the entire default template while still substituting
{{NAME}} and {{SOUL}} placeholders.
Advanced Settings UI:
- System Prompt textarea below Soul (monospace, shows built-in
default as placeholder when empty)
- Documents {{NAME}} and {{SOUL}} placeholder usage
- Reset to default button when custom prompt is active
- Saves on blur to AppConfig (persisted in localStorage)
Lemon
64c1cf3642