fix: implicitly allow dropping in a component that has a default slot + other slot fixes

[ruchamahabal]

The old canHaveChildren() check only allowed you to drop a component inside another one if:

1. It's a root/container block
2. It had some slot enabled (even default)

But in actual Vue components, wrapping a component around the other actually renders correctly if it just has a default slot. No explicit slot passing is required in this case.
This PR fixes that behaviour. You can now drop any component in another component if it has a default slot. For any other slot, you have to enable that slot from the Properties panel. This makes composition easier.

drop.mp4

Also fixes:

• Stale slot options dropdown in properties panel even when you edit the Vue file in code. (needed a reload before this)

• This PR also caches slots per component so that lookup becomes easier

• Duplicate default slots shown in selector (all unnamed slots were considered as default branching from v-if regex)

  Before	After

[greptile-apps]

Confidence Score: 5/5

Safe to merge; the change correctly pre-warms slot caches and the only concern is duplicate network requests under concurrent construction of the same custom-component block.

The logic is sound: startup pre-warming via registerCustomComponentPaths covers the common path, and the Block constructor's void getComponentSlots(…) is a belt-and-suspenders guard. Duplicate fetches are a minor efficiency concern, not a correctness one.

frontend/src/utils/components.ts — getComponentSlots in-flight deduplication.

_{Reviews (2): Last reviewed commit: "fix: register custom component slots & t..." | Re-trigger Greptile}