fix: surface generation errors and guide initial model setup

[mrsibe]

What does this PR do?

Surfaces actionable generation errors and expands first-run onboarding with embedding and chat model setup.

Why?

Users previously saw a generic failure or no feedback when generation failed, while the actual reason was only logged to the console. New users also had no model setup steps in onboarding, despite embedding and chat models serving different purposes.

What changed?

• Display specific errors for Anki card, quiz, and mind map generation, including rejected background IPC calls.
• Display errors when sending questions, retrying answers, or continuing answers; preserve existing inline feedback for streaming failures.
• Add embedding setup and optional chat setup after language selection, reusing the existing model configuration UI.
• Allow users to defer model setup; persist configurations when continuing and discard skipped connection edits.
• Display onboarding and embedding download errors in the UI.
• Add English and Simplified Chinese copy and regression guards for feedback and onboarding entry points.

How was this tested?

• npm test: all 490 tests passed.
• npm run build: passed, including the full npm run typecheck gate.
• Targeted ESLint checks: no errors; one existing Hook dependency warning in NotePanel.tsx.
• git diff --check: passed.
• Electron interaction, real model downloads, and end-to-end generation have not been manually verified.

Screenshots / recordings

Not captured in this environment. The onboarding UI changes still need visual and desktop interaction verification.

Checklist

• I have reviewed my own changes.
• npm run typecheck passes.
• npm run build passes.
• I have tested the affected user workflow.
• I have not included unrelated changes.
• I have updated documentation when necessary. (No separate documentation changes were needed; onboarding guidance is included in the UI.)

Desktop / build changes

• Not applicable
• npm run build:unpack passes.
• npm run smoke:packaged passes.