fix: harden editor and sync edge cases

[ShiroKSH]

Summary

• Prevent active-link ranges from merging adjacent links with different attributes.
• Preserve mixed task and plain list items when toggling list type.
• Preserve trailing text after inline Markdown images.
• Prevent duplicate debounced saves and make desktop persistence best-effort.
• Reject failed asset upload and download responses before updating sync state.
• Remove the Unix-only CLI postbuild step so Windows CLI builds succeed.

Validation

• pnpm test: 175 tests passed.
• pnpm build passed.
• pnpm check passed.
• pnpm typecheck passed.

Scope

This change keeps the existing sync and workspace contracts intact. Server-side authentication, ownership, path validation, and conflict resolution remain separate follow-up work.

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