Add thumbs up/down feedback for Rundown briefing quality

[shellen]

Problem

Users have no way to signal whether a generated Rundown briefing was good or bad. This feedback could be used to improve prompt tuning or provider selection over time.

What needs to happen

• Add thumbs up/down buttons to the briefing footer (next to refresh)
• Store feedback locally (localStorage or file-backed)
• Design what happens with the data: prompt tuning, provider switching, analytics
• Consider: should negative feedback trigger a re-generation with adjusted prompt?

Context

The briefing is generated by generateBriefing() in src/review.ts using whichever LLM provider is configured. The briefing UI is in viewer/04-article.js (renderBriefing).