fix(desktop): explain workspace binding conflicts

[Sun-sunshine06]

Summary

Fixes #286.

• Keep the one-workspace-per-design guard, but return the conflicting design name in the error.
• Add actionable guidance so the UI toast tells users to choose another folder or change the other design's workspace first.
• Preserve the existing boolean conflict helper while adding a conflict lookup for richer messages.

Validation

• pnpm exec biome check apps/desktop/src/main/design-workspace.ts apps/desktop/src/main/design-workspace.test.ts .changeset/workspace-conflict-guidance.md
• pnpm --dir apps/desktop exec vitest run src/main/design-workspace.test.ts
• pnpm --dir apps/desktop typecheck
• pnpm --dir apps/desktop test
• pnpm typecheck

[github-actions]

Review mode: initial

Findings

No issues found.

Summary

• The PR correctly addresses issue #286 by improving the workspace conflict error message to include the conflicting design name and actionable guidance.
• Refactored checkWorkspaceConflict to use a new findWorkspaceConflict helper that returns the conflicting Design object, enabling richer error messaging without duplicating the lookup logic.
• Updated the existing test to verify the new, more descriptive error message.
• Added a changeset for the user-visible change.
• Code follows project conventions: no new dependencies, no security concerns, no silent fallbacks, unchanged public API surface, Biome-compatible formatting.

Testing

Existing test passes with updated expectations. No additional test scenarios are needed for this straightforward change.

Open-CoDesign Bot