Prevent layout shift in Popular Threads loading state

[plebe1us]

The Popular Threads box currently resizes after content loads, causing a jarring layout shift. We should reserve space during loading by applying a min-height based on expected content (max 8 posts, ~2 rows of 4 cards on desktop). The loading text should also be left-aligned instead of centered to match the rest of the content.

Additionally, the boards list doesn't need a loading state since it uses hardcoded default data that's available synchronously.

[plebe1us]

c1a66ca