feat: add export and import actions to slash command menu (#1165)

[sw-factory-automations]

Closes #1165

What

Adds "Export as Markdown" and "Import Markdown" options to the editor's slash command menu (/ typeahead), improving discoverability of these low-usage features.

How

• Added optional onExport and onImport callback props to SlashCommandPlugin. When provided, the menu shows "Export as Markdown" (Download icon) and "Import Markdown" (Upload icon) at the bottom of the options list, after existing block insertion commands.
• Wired the callbacks in PageViewClient:
  • Export reuses exportEditorToMarkdown + downloadMarkdown from markdown-utils.ts (same flow as the page menu).
  • Import reuses the useMarkdownImport hook (same file picker flow as the page menu), with a hidden <input type="file"> rendered alongside the editor.
• Both options are filterable by typing (e.g., /export, /import).
• The Editor component threads the callbacks via onSlashExport/onSlashImport props. Callers that don't provide them (database view, preview mode) simply don't show the options.
• Page menu keyboard shortcuts (⌘⇧E for export) continue to work independently.

Testing

• pnpm lint — 0 errors
• pnpm typecheck — passes
• pnpm test — 144 files, 1975 tests passed
• Storybook: added FilteredExportImport story showing the two new options filtered by search
• Visual regression: new baseline committed, all stories pass
• E2E: demo editor tests pass (11/11). Authenticated E2E tests have pre-existing auth fixture failures unrelated to this change.

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Project	Deployment	Actions	Updated (UTC)
memo	Building	Preview, Comment	May 21, 2026 2:59pm