feat: add studio timeline editing (#390)

## Summary

Add the actual Studio timeline editing layer on top of the preview/runtime foundation.

This PR includes:

- drag-to-move clips across time and tracks
- left/right resize handles with media-aware trim persistence
- edge auto-scroll and edge track creation while dragging
- selector-based source patching for `data-start`, `data-duration`, `data-track-index`, `z-index`, and media trim attributes
- timeline UI cleanup, theming, hover/drag states, and the `Copy Prompt` action

## Why This PR Is Separate

This is the user-facing editing behavior. It depends on the preview/runtime fixes in the base PR, but it is much easier to review once that plumbing is isolated.

## Verification

- `bun run --filter @hyperframes/studio test`
- `bun run --filter @hyperframes/studio typecheck`
- `bunx oxlint packages/studio/src/App.tsx packages/studio/src/components/nle/NLELayout.tsx packages/studio/src/player/components/EditModal.tsx packages/studio/src/player/components/Timeline.tsx packages/studio/src/player/components/TimelineClip.tsx packages/studio/src/player/components/timelineEditing.ts packages/studio/src/player/components/timelineEditing.test.ts packages/studio/src/player/components/timelineTheme.ts packages/studio/src/player/components/timelineTheme.test.ts packages/studio/src/utils/sourcePatcher.ts packages/studio/src/utils/sourcePatcher.test.ts`
- `bunx oxfmt --check packages/studio/src/App.tsx packages/studio/src/components/nle/NLELayout.tsx packages/studio/src/player/components/EditModal.tsx packages/studio/src/player/components/Timeline.tsx packages/studio/src/player/components/TimelineClip.tsx packages/studio/src/player/components/timelineEditing.ts packages/studio/src/player/components/timelineEditing.test.ts packages/studio/src/player/components/timelineTheme.ts packages/studio/src/player/components/timelineTheme.test.ts packages/studio/src/utils/sourcePatcher.ts packages/studio/src/utils/sourcePatcher.test.ts`

## Browser Proof

- verified timeline drag / resize / trim flows in Studio with `agent-browser`
- verified preview hot-refresh behavior without iframe remount flashes

## Stack

- depends on #389
- followed by `fix: smooth scrubber end seeking`

[result.mp4 <span class="graphite__hidden">(uploaded via Graphite)</span> <img class="graphite__hidden" src="https://app.graphite.com/user-attachments/thumbnails/ca71c177-5042-468d-906f-b353938f40f8.mp4" />](https://app.graphite.com/user-attachments/video/ca71c177-5042-468d-906f-b353938f40f8.mp4)

packages/studio/src/player/components/timelineTheme.test.ts

@@ -0,0 +1,56 @@
+import { describe, expect, it } from "vitest";
+import {
+  getClipHandleOpacity,
+  getRenderedTimelineElement,
+  getTimelineTrackStyle,
+} from "./timelineTheme";
+
+describe("getTimelineTrackStyle", () => {
+  it("reuses heading styles for heading tags", () => {
+    expect(getTimelineTrackStyle("h2").accent).toBe(getTimelineTrackStyle("h1").accent);
+  });
+
+  it("falls back for unknown tags", () => {
+    expect(getTimelineTrackStyle("custom-tag").accent).toBe("#3CE6AC");
+  });
+});
+
+describe("getClipHandleOpacity", () => {
+  it("hides handles at rest", () => {
+    expect(getClipHandleOpacity({ isHovered: false, isSelected: false, isDragging: false })).toBe(
+      0,
+    );
+  });
+
+  it("prioritizes dragging over hover and selection", () => {
+    expect(getClipHandleOpacity({ isHovered: true, isSelected: true, isDragging: true })).toBe(
+      0.95,
+    );
+  });
+});
+
+describe("getRenderedTimelineElement", () => {
+  it("keeps non-dragged clips unchanged", () => {
+    const element = { id: "a", tag: "div", start: 1, duration: 2, track: 0 };
+    expect(
+      getRenderedTimelineElement({
+        element,
+        draggedElementId: "b",
+        previewStart: 2,
+        previewTrack: 1,
+      }),
+    ).toEqual(element);
+  });
+
+  it("moves the actual dragged clip to the preview position", () => {
+    const element = { id: "a", tag: "div", start: 1, duration: 2, track: 0 };
+    expect(
+      getRenderedTimelineElement({
+        element,
+        draggedElementId: "a",
+        previewStart: 2.4,
+        previewTrack: 3,
+      }),
+    ).toEqual({ ...element, start: 2.4, track: 3 });
+  });
+});