Make the Inspector UI more smooth

[shouya]

1. Blocking on fetch: rss-funnel is essentially a reverse proxy. There is little we can do to make the upstream respond faster. But we can certainly further avoid blocking the ui and perhaps show a nicer loading animation.
2. Another source of choppiness is the occasional GC pauses. I need to look into the root cause of this.
3. Add a debouncing logic for fetching to avoid issuing repeated heavy actions.
4. Make sure only one fetch request is handled. If a new one comes before the old one finishes, cancel it.
5. Cache more aggressively, add the cache for 5xx and 4xx errors as well.

[tillcash]

Additionally, please consider implementing a dark mode feature.

[shouya]

I will consider a dark mode after the UI rewrite. The current UI code was written as a poc without considering much maintainability. I am planning to rewrite it fully before adding any features to it.