fix: defects for token/retry/loading UX

[gildesmarais]

Summary

• tighten token rejection detection so non-token forbidden/policy failures do not clear saved token or trigger token-recovery UX
• only show manual strategy-switch recovery when it is actionable (faraday -> browserless)
• update converting-state copy to reflect actual behavior: result first, preview after
• add regression coverage for non-token 403 behavior

Scope

• Workstream B follow-up only
• no broad refactors

Validation

• cd frontend && npm run test:unit -- --run src/tests/App.test.tsx

Notes

• docs workspace in this environment has no git remote configured, so only the html2rss-web PR is opened here.

[Copilot]

Pull request overview

This PR refines the frontend’s error/UX handling around access-token failures and manual retry strategy prompts, aligning UI behavior with actual conversion flow and adding a regression test to prevent token-clearing on non-token 403s.

Changes:

• Tightens access-token rejection detection and gates token-recovery UX to token-related failures.
• Shows the manual “Try X instead” strategy-switch action only when switching from faraday → browserless is actionable.
• Updates converting-state copy and adds regression coverage for non-token forbidden behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
frontend/src/components/AppPanels.tsx	Updates token input field configuration and loading-state copy.
frontend/src/components/App.tsx	Refines token error detection and strategy-switch eligibility logic.
frontend/src/__tests__/App.test.tsx	Updates copy assertion and adds regression test for non-token forbidden failures.