[P1] Add actionable failure states for import, restore, and thumbnail rendering

[nkanf-dev]

Summary

Surface clear, actionable failure states in the UI for document import, workspace restore, and thumbnail generation.

Why

Today several failure paths either only log to the console or collapse into generic error messaging, which makes the product feel unreliable and opaque.

Current evidence

• Import and restore flows rely heavily on silent or console-only error handling.
• Thumbnail rendering currently has no meaningful user-facing error recovery path.

Scope

• Define explicit user-facing states for failed import, failed restore, and failed thumbnail render.
• Provide actionable next steps where possible (retry, reload, re-import, dismiss).
• Ensure failure states do not leave stale loading indicators.
• Add test coverage for visible failure behavior.

Acceptance criteria

• Users receive clear feedback when core flows fail.
• Console-only failure handling is reduced for primary workflows.
• Failure states recover cleanly without full app refresh when feasible.

Out of scope

• Full internationalization of all error copy.