List: groups expanded by default when collapsible=true

[jerrythomas]

Description

When using <List collapsible />, all groups are expanded on mount. This is incorrect — the "expand all on mount" behavior should only apply when collapsible=false (groups act as fixed section headers). When collapsible=true, groups should start collapsed by default, letting the user expand them.

Root Cause

packages/ui/src/components/List.svelte lines 88–92:

```js
$effect(() => {
for (const [, proxy] of wrapper.lookup) {
if (proxy.hasChildren) proxy.expanded = true
}
})
```

This runs unconditionally regardless of the collapsible prop.

Expected Behavior

• collapsible=false: all groups expanded (current behaviour — correct)
• collapsible=true: all groups collapsed on mount; user expands individually

Affected File

packages/ui/src/components/List.svelte