Two-way Notion integration (import + export)

Turned the connected-but-inert Notion link into working features. Both directions go through a shared page picker (/v1/search).

Added

• lib/notion-shared.ts — client+server-safe DTOs (NotionPage, NotionImportItem) so block↔card mapping stays UI-side.
• lib/notion.ts — getNotionToken() (reads the stored token via the admin client), searchNotionPages(), cardsToNotionBlocks() (all 9 card kinds → paragraphs / bookmarks / callouts; data-URL images are marked, not exported), createNotionExport() (creates a sub-page, chunks >100 blocks), and fetchNotionPageItems() (paginated block read → neutral items).
• Routes — GET /api/integrations/notion/pages, POST …/export (cards in the body → new Notion page, returns its URL), POST …/import (page id → import items).
• Canvas.addCards() — bulk-places pre-built cards at the viewport center (incoming x/y treated as relative offsets), persisted + synced like any add.
• IntegrationsPanel — when Notion is connected, the content-source row shows Import and the export row shows Export, both opening a NotionPicker modal. Import drops cards on the canvas and closes the drawer; export opens the new Notion page. Skeleton, empty ("add this integration under Connections"), and error states included.

Notes

• Notion-hosted image URLs are time-limited signed links, so imported images may expire; external image URLs persist.
• Export requires picking a parent page (Notion's API can't create top-level workspace pages), which is why the picker is shared with import.