refactor(muya): type mutation sources with ChangeSource - #5811

Summary

Mutation-source parameters currently accept arbitrary strings despite having only user and api values. Export ChangeSource from types.ts and use it throughout editor operations, history, block mutations, and their callers.

The enum retains the existing string values and defaults. TypeScript callers must pass ChangeSource.User or ChangeSource.Api.

Type of change

• Breaking change (TypeScript API tightening)

Test plan

• Full Muya unit suite; desktop/Muya typecheck; Muya and root ESLint.
• Existing regression tests cover operation dispatch, undo/redo and task-checkbox updates; new tests would duplicate those contracts.
• Isolated worktree tests used a temporary Vite filesystem allowlist to reuse installed dependencies; the test configuration is not part of the PR.
• No visual change; screenshots and manual platform UI checks are not applicable.

By submitting this pull request, I confirm that my contribution is made under the terms of the MIT license.