Vaults and Activity block the whole page on a spinner instead of a skeleton

## Context

The Vaults and Activity pages hide their whole content behind a spinning loader while data loads.

## Changes

**Now:** both pages gate the entire list on the same core-ui `Loader` — a continuous 1.4s rotate plus stroke-dash animation with no static fallback (`packages/babylon-core-ui/src/components/Loader/Loader.tsx`, `Loader.css:24-32`).

- `services/vault/src/components/pages/VaultsPage.tsx:163-167`
- `services/vault/src/components/pages/Activity.tsx:29-31`

**Expected:** a skeleton that keeps the page shape while loading. No `Skeleton` component exists in core-ui today, so this starts with adding one.
**Where:** `VaultsPage.tsx:163-167`, `Activity.tsx:29-31`, plus a new component in `packages/babylon-core-ui`

Out of scope: the inline `` used for per-row pending states (`VaultsActiveSection.tsx:91`, `VaultsLifecycleSections.tsx:257,435,703`) — those mark a single row, not the page.

Related: the Liquidation Analysis loading state is #2418.

Original report

[Slack thread](https://babylonlabsworkspace.slack.com/archives/C07EG11U45A/p1788267860997369)

> UX complaint about the spinner
> I find it really distracting and doesnt add much value IMO

Start with the loading branches in services/vault/src/components/pages/VaultsPage.tsx:163-167 and Activity.tsx:29-31, then review the existing Loader in packages/babylon-core-ui/src/components/Loader. Add the core-ui Skeleton component and use it to preserve each page’s shape while data loads; leave the per-row loaders unchanged. Done means both pages show skeleton content instead of blocking on the spinner.

Written by the indexing model from the issue text.