Enable Android cursor drag positioning

[Agentriel]

Closes #443

IssueHunt bounty: https://issuehunt.io/r/BoostIO/BoostNote-App/issues/443

Summary

• add an Android-only long-press cursor drag handler for the mobile CodeMirror editor
• move the cursor with coordsChar/setCursor while dragging after the long press activates
• clean up touch/context-menu listeners when the editor unmounts, while leaving normal tap/scroll gestures alone before activation

Validation

• npm exec --package prettier@2.5.1 -- prettier --check src/mobile/lib/codeMirrorTouchCursor.ts src/mobile/components/pages/DocEditPage.tsx
• git diff --check

I could not run the full project test suite locally because installing the existing dependency tree exhausted the /tmp tmpfs in this environment.

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