feat: add Studio current-frame capture (#565)

## Problem

Closes #555. Studio users could inspect the preview, but there was no first-class way to capture the current rendered frame as an image.

## What this fixes

- Adds a `Capture` action to the Studio header toolbar so it does not cover the video preview.
- Downloads the current composition frame as a PNG using the current player time.
- Extends the existing thumbnail route and Studio/CLI thumbnail generators with an explicit PNG format path while preserving JPEG thumbnails for existing previews.
- Adds URL/filename utility coverage plus thumbnail route coverage for PNG requests.

## Root cause

Studio already had frame thumbnail generation, but the API path was JPEG-oriented and the editor UI only used it for previews. There was no current-frame capture affordance wired to the player state.

## Verification

### Local

- `bun run --filter @hyperframes/core test src/studio-api/routes/thumbnail.test.ts`
- `bun run --filter @hyperframes/studio test src/utils/frameCapture.test.ts src/player/components/PlayerControls.test.ts`
- `bun run --filter @hyperframes/studio typecheck`
- `bun run --filter @hyperframes/core typecheck`
- `bun run --filter @hyperframes/cli typecheck`
- `bunx oxlint packages/cli/src/server/studioServer.ts packages/core/src/studio-api/routes/thumbnail.test.ts packages/core/src/studio-api/routes/thumbnail.ts packages/core/src/studio-api/types.ts packages/studio/src/App.tsx packages/studio/src/icons/SystemIcons.tsx packages/studio/vite.config.ts packages/studio/src/utils/frameCapture.ts packages/studio/src/utils/frameCapture.test.ts`
- `bunx oxfmt --check packages/cli/src/server/studioServer.ts packages/core/src/studio-api/routes/thumbnail.test.ts packages/core/src/studio-api/routes/thumbnail.ts packages/core/src/studio-api/types.ts packages/studio/src/App.tsx packages/studio/src/icons/SystemIcons.tsx packages/studio/vite.config.ts packages/studio/src/utils/frameCapture.ts packages/studio/src/utils/frameCapture.test.ts`
- `git diff --check`

### Browser
<img width="1027" height="910" alt="image" src="https://github.com/user-attachments/assets/71973af4-0279-4074-9
<img width="1026" height="902" alt="Screenshot 2026-04-29 at 16 17 22" src="https://github.com/user-attachments/assets/a32e1c19-b793-40b9-82f8-de8bbb11f123" />
060-130839a2d419" />

packages/studio/src/components/sidebar/LeftSidebar.tsx

@@ -35,6 +35,7 @@ interface LeftSidebarProps {
   codeChildren?: ReactNode;
   onLint?: () => void;
   linting?: boolean;
+  onToggleCollapse?: () => void;
 }
 
 export const LeftSidebar = memo(function LeftSidebar({
@@ -57,6 +58,7 @@ export const LeftSidebar = memo(function LeftSidebar({
   codeChildren,
   onLint,
   linting,
+  onToggleCollapse,
 }: LeftSidebarProps) {
   const [tab, setTab] = useState<SidebarTab>(getPersistedTab);
 
@@ -122,6 +124,30 @@ export const LeftSidebar = memo(function LeftSidebar({
         >
           Assets
         </button>
+        {onToggleCollapse && (
+          <button
+            type="button"
+            onClick={onToggleCollapse}
+            className="mx-1 my-1 flex h-7 w-7 flex-shrink-0 items-center justify-center rounded-md border border-transparent text-neutral-500 transition-colors hover:border-neutral-800 hover:bg-neutral-900 hover:text-neutral-300"
+            title="Hide sidebar"
+            aria-label="Hide sidebar"
+          >
+            <svg
+              width="14"
+              height="14"
+              viewBox="0 0 24 24"
+              fill="none"
+              stroke="currentColor"
+              strokeWidth="1.5"
+              strokeLinecap="round"
+              strokeLinejoin="round"
+              aria-hidden="true"
+            >
+              <path d="m14 7-5 5 5 5" />
+              <path d="M19 4v16" />
+            </svg>
+          </button>
+        )}
       </div>
 
       {/* Tab content */}