Generate: improve empty, loading, and error states

[WalksWithASwagger]

Summary

• Split Generate route status into distinct validation, loading, generation/backend failure, and notice states.
• Added explicit empty/loading/unavailable panels for library references, media references, selected references, prompt preview, and route loading.
• Preserved safe backend detail handling while avoiding resolved filesystem paths in missing prompt-file errors.
• Extended portal smoke coverage for validation refusal before /api/regen and missing prompt-preview errors.

Closes #296

Verification

• npm run frontend:verify (passes; existing fast-refresh warnings in shared UI files remain)
• python3 -m pytest tests/test_server_generate.py tests/test_server_endpoints.py -q (37 passed)
• npm run e2e:portal (passes)
• git diff --check (passes)

[github-actions]

Agentic PR Review

Verdict: needs-human

Checks:

• Linked issue via Closes #... or Refs #...: yes
• Issue acceptance criteria found: 13
• PR verification section present: yes
• PR acceptance self-check present: no
• Diff within v1 limits: yes

This PR is not blocked from human review, but the agentic gate is not satisfied.

[github-actions]

Agentic PR Review

Verdict: needs-human

Checks:

• Linked issue via Closes #... or Refs #...: yes
• Issue acceptance criteria found: 13
• PR verification section present: yes
• PR acceptance self-check present: no
• Diff within v1 limits: yes

This PR is not blocked from human review, but the agentic gate is not satisfied.

[github-actions]

Agentic PR Review

Verdict: needs-human

Checks:

• Linked issue via Closes #... or Refs #...: yes
• Issue acceptance criteria found: 13
• PR verification section present: yes
• PR acceptance self-check present: no
• Diff within v1 limits: yes

This PR is not blocked from human review, but the agentic gate is not satisfied.

[github-actions]

Agentic PR Review

Verdict: needs-human

Checks:

• Linked issue via Closes #... or Refs #...: yes
• Issue acceptance criteria found: 13
• PR verification section present: yes
• PR acceptance self-check present: no
• Diff within v1 limits: yes

This PR is not blocked from human review, but the agentic gate is not satisfied.