feat(templates): add markdown template management

[murongg]

Summary

• Add a lightweight Markdown template system with built-in and custom templates, editable Markdown source, preview mode, and template variable rendering.
• Store template bodies as individual .md files in the native template directory while keeping only the template list/index in settings.json.
• Add a dedicated Templates settings section with a two-pane list/editor layout and localized UI text.
• Support creating Markdown files from templates and saving an existing Markdown file as a template from the file tree.

Validation

• pnpm --filter @markra/desktop test src/lib/templates.test.ts src/lib/tauri/file.test.ts src/lib/settings/app-settings.test.ts src/components/MarkdownFileTreeDrawer.test.tsx src/components/SettingsSections.test.tsx -- --runInBand
• pnpm --filter @markra/desktop test src/App.test.tsx -- --runInBand
• pnpm --filter @markra/shared test src/i18n/index.test.ts
• pnpm --filter @markra/desktop typecheck:test
• pnpm --filter @markra/shared typecheck:test
• cargo test in apps/desktop/src-tauri
• pnpm --filter @markra/desktop build (passes; Vite reports the existing large chunk warning)
• pnpm --filter @markra/shared build

Closes #44