Implement whiteboard library templates

[hweihwang]

Summary

Implements Whiteboard library templates end to end:

• user save-as library templates via POST /apps/whiteboard/library/template
• safer personal library persistence with server-owned template paths
• organization library template admin upload/list/delete
• appdata-backed global .excalidrawlib storage
• custom template provider so organization libraries appear in the existing New whiteboard picker
• template-created whiteboards stay empty on canvas but embed copied libraryItems
• malformed libraries are skipped/logged instead of breaking the full library
• frontend save-as dialog, admin settings UI, embedded board-library loading, and clearer copy

Dependency

Depends on nextcloud-deps/excalidraw#16 for the Excalidraw library menu hook.

Draft note: package.json currently points at local file: dependencies for the patched Excalidraw packages so this branch can be tested locally. Before merge this must be replaced by the published package/version or another shippable package reference from the Excalidraw dependency PR.

Validation

• npm run lint passed locally
• npm run stylelint passed locally
• composer lint passed locally
• composer cs:check passed locally
• npm test passed locally
• npm run build passed locally
• git diff --check passed locally
• Browser E2E passed locally: admin upload, picker create-from-template, empty canvas with embedded library items, user save-as template, picker shows saved user template