feat: add configurable chat prompt features

[hikariming]

Summary

• add persistent prompt feature config with builtin defaults and migration coverage in Tauri
• add settings UI and helper logic for editable chat prompt features and quick actions
• prefill new prompt templates with {text} and add inline help for the placeholder

Test Plan

• npm test -- --run src/components/features/SettingsDialog.test.tsx src/components/features/ArticleChatAssistant.test.tsx src/components/features/AssistantSidebarShell.test.tsx src/lib/promptFeatures.test.ts
• cargo test --manifest-path textlingo-desktop/src-tauri/Cargo.toml --test prompt_feature_config_test -- --nocapture