Support dropped text files as new docs

[dicnunz]

Summary

• add support for dropping .md, .txt, .html, and .htm files to create new docs
• create dropped-file docs in the target workspace, folder, or current content-manager context
• keep the existing internal folder/doc drag-and-drop behavior intact

Fixes #1151

Reproduction

The existing cloud drag-and-drop handlers only read BoostNote internal doc/folder transfer data. Dropping a local text file onto a workspace, folder, or document list does not create a document from the file content.

Changes

This adds a file-drop path to the shared cloud DnD hook. Supported text files are read with FileReader, sent through the existing createDoc API, titled from the file name, and inserted into the local doc plus parent workspace/folder maps after creation.

Tests

• npm ci
• ./node_modules/.bin/eslint src/cloud/lib/hooks/sidebar/useCloudDnd.ts src/cloud/lib/hooks/sidebar/useCloudSidebarTree.tsx src/cloud/components/ContentManager/index.tsx --ext .ts,.tsx
• ./node_modules/.bin/tsc --noEmit
• npm run test-api -- --runInBand
• npm run lint
• git diff --check

Bounty

Submitted for the $50 IssueHunt bounty: https://issuehunt.io/repos/74213528/issues/1151

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}