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/utils/frameCapture.ts

@@ -0,0 +1,38 @@
+export interface FrameCaptureRequest {
+  projectId: string;
+  compositionPath: string | null;
+  currentTime: number;
+  origin?: string;
+}
+
+function normalizeCompositionPath(compositionPath: string | null): string {
+  return compositionPath && compositionPath !== "master" ? compositionPath : "index.html";
+}
+
+export function buildFrameCaptureUrl({
+  projectId,
+  compositionPath,
+  currentTime,
+  origin = window.location.origin,
+}: FrameCaptureRequest): string {
+  const compPath = normalizeCompositionPath(compositionPath);
+  const url = new URL(
+    `/api/projects/${encodeURIComponent(projectId)}/thumbnail/${encodeURIComponent(compPath)}`,
+    origin,
+  );
+  url.searchParams.set("t", Math.max(0, currentTime).toFixed(3));
+  url.searchParams.set("format", "png");
+  url.searchParams.set("v", String(Date.now()));
+  return url.toString();
+}
+
+export function buildFrameCaptureFilename(compositionPath: string | null, currentTime: number) {
+  const compPath = normalizeCompositionPath(compositionPath);
+  const base =
+    compPath
+      .split("/")
+      .pop()
+      ?.replace(/\.html$/i, "") || "frame";
+  const frameTime = Math.max(0, currentTime).toFixed(3).replace(".", "-");
+  return `${base}-${frameTime}s.png`;
+}