Fix Vim Ctrl-C copy shortcut

[dicnunz]

Fixes #773.

IssueHunt bounty: https://issuehunt.io/r/BoostIO/BoostNote-App/issues/773

Reproduction

The issue report describes Vim keymap mode on Windows/Linux where Ctrl-C exits Vim insert mode instead of copying the selected editor text to the clipboard. A maintainer pointed to the old Boostnote fix and suggested adding an extra CodeMirror key binding in the editor setup.

Changes

• Adds a shared CodeMirror Ctrl-C extra key for Vim keymap mode.
• Calls the browser copy command before returning CodeMirror.Pass, preserving the existing Vim key handling after the copy attempt.
• Merges the default editor extra keys with per-editor config extra keys so the copy binding is not lost when editor configs provide their own extraKeys.

Verification

• ./node_modules/.bin/eslint src/cloud/lib/editor/components/CodeMirrorEditor.tsx --ext .tsx
• ./node_modules/.bin/tsc --noEmit
• npm run lint
• git diff --check

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