UI: match count to bottom, remove overview CTAs, calmer upload status

[aperson30]

Addresses several UI asks. Three landed here; two more (CT-count accuracy, shuffle speed) are noted below and I'll follow up.

In this PR

• Match count → bottom of screen. The " cases match" count moved out of the library header into a fixed bar at the bottom-left, so it's out of the way while filtering.
• Removed the Browse Dataset / Upload Dataset buttons from the landing/overview hero (and the now-unused navigate).
• Calmer multi-upload status. Replaced the per-scan spinning wheel with a subtle pulsing dot, so uploading several scans no longer shows a wall of spinners; the single spinner stays in the batch summary bar.

Typecheck passes (tsc -b, verified on the server toolchain).

Follow-ups (need a decision / investigation)

• CT-volumes number (36,390): it's a hardcoded TARGETS.ctVolumes in LandingPage. I didn't want to guess a new wrong number on a public page — what's the correct count, or should I make it dynamic (fetch the real dataset total)?
• Shuffle load speed: needs a look at the shuffle load path to find the bottleneck; separate change.