test: add render coverage for Skeleton.tsx

[tosin-zoffun]

Changes

• [Frontend] Skeleton.tsx exports multiple skeleton variants but has no unit test verifying each renders the expected number of placeholder rows #1032: the issue describes Skeleton.tsx exporting several variants with row-count props (plus a mentioned but already-dead StreamListSkeleton). As it exists on main today, Skeleton.tsx exports exactly one component — a single className-only placeholder <div>, no row-count prop, no StreamListSkeleton or any other named variant anywhere in the codebase (checked with a repo-wide grep). There's nothing matching the multi-variant "assert placeholder count equals the row prop" description to test without inventing that API from scratch, which would be adding a feature, not testing one that exists.
• Added Skeleton.test.tsx covering the one export that does exist: renders exactly one placeholder element, applies the default animate-pulse/rounded-md styling, and correctly merges a caller-supplied className.

Test plan

• npx vitest run src/components/ui/Skeleton.test.tsx — 4 passed
• npx vitest run (full suite) — 186 passed; the 2 pre-existing failing suites (useIncomingStreams.test.ts and one other, both a .ts-file-using-JSX esbuild error) fail identically on main before this change, confirmed via git stash -u — unrelated to this change, out of scope

Closes #1032