fix(studio): restore golden-branch timeline behaviors dropped by the stack rebuild

The Studio stack rebuild (#2291) landed the remaining NLE layers but dropped
or regressed several final-wave behaviors from the reviewed studio-dnd stack,
and never repaired the stale timelineZones.ts that #2279 introduced. Restores:

- TimelineRuler: sticky under vertical scroll, full-height gridlines removed
  (beat lines only), frame-number tick labels via a persisted timeDisplayMode
  store preference (PlayerControls toggle now store-backed)
- timelineZones: stable track lanes — lane = authored data-track-index
  ascending; z is paint order only (replaces the stale z-driven lane pack,
  which broke track insert-band commits that contractually depend on it)
- persistTimelineBatchEdit: a batch member whose patch is a no-op (attributes
  already at target values, e.g. in a track-insert renumber) is skipped
  instead of aborting and rolling back the whole batch — this alone made
  new-track creation (incl. the top insert band) fail silently
- useTimelineStackingSync: unresolvable clips read as NaN again so
  timelineStackingSync's Number.isFinite exclusion contract holds (z=0
  fabrications skewed stacking boundaries)
- timelineAssetDrop: drops land on the drop track (no overlap bump to
  max-track+1), data-hf-id stamped, audio gets data-volume
- timing edits: soft-reload the server's rewritten GSAP script instead of a
  full iframe remount (no all-clips flash on move/resize); full reload only
  when no scriptText or the soft path can't apply, and one full reload when a
  group edit touches non-active files (new hooks/timelineTimingSync.ts)
- duration: content-driven grow-AND-shrink on move/resize/delete, synced
  optimistically to the store and the live root data-duration at release
  (was a grow-only ratchet; shrink never updated the readout)

New UX: sidebar asset click opens a compact non-modal preview over the canvas
(dismiss on outside click, Escape, playback, or seek), and clicking an
already-added asset reveals its clip in the timeline (smooth minimal scroll
to its time and lane; vertical-only in fit zoom).

Verified by pointer-driving a real project: sticky ruler + gridline removal,
no iframe remount on move/resize (marker survives, GSAP tween positions
rewritten in place), duration readout 40->37->40 on shrink/stretch, and
top-insert-band track creation renumbering lanes correctly on disk.

packages/studio/src/hooks/timelineEditingHelpers.test.ts

@@ -2,8 +2,11 @@
 import { afterEach, describe, expect, it, vi } from "vitest";
 import {
   applyTimelineStackingReorder,
+  buildTimelineMoveTimingPatch,
   deleteSelectedKeyframes,
   extendRootDurationIfNeeded,
+  persistTimelineBatchEdit,
+  type PersistTimelineBatchChange,
 } from "./timelineEditingHelpers";
 import type { TimelineElement } from "../player/store/playerStore";
 import { usePlayerStore } from "../player/store/playerStore";
@@ -108,6 +111,91 @@ describe("extendRootDurationIfNeeded", () => {
   });
 });
 
+describe("persistTimelineBatchEdit", () => {
+  const SOURCE = `<div id="root"><video id="a" class="clip" data-start="1" data-track-index="0"></video><video id="b" class="clip" data-start="2" data-track-index="1"></video></div>`;
+
+  function batchInput(changes: PersistTimelineBatchChange[], writes: Array<[string, string]>) {
+    return {
+      projectId: "p1",
+      activeCompPath: "index.html",
+      label: "Move timeline clips",
+      changes,
+      writeProjectFile: async (path: string, content: string) => {
+        writes.push([path, content]);
+      },
+      recordEdit: async () => {},
+      domEditSaveTimestampRef: { current: 0 },
+      pendingTimelineEditPathRef: { current: new Set<string>() },
+    };
+  }
+
+  function stubReadFileContent(content: string) {
+    vi.stubGlobal(
+      "fetch",
+      vi.fn(async () => ({
+        ok: true,
+        json: async () => ({ content }),
+      })),
+    );
+  }
+
+  afterEach(() => {
+    vi.unstubAllGlobals();
+  });
+
+  it("skips no-op members instead of aborting the batch (track-insert renumber)", async () => {
+    // A track-insert renumber can include a member whose attributes already
+    // hold the target values — its patch is string-identical. The batch must
+    // skip it and still persist the members that DID change.
+    stubReadFileContent(SOURCE);
+    const writes: Array<[string, string]> = [];
+
+    await persistTimelineBatchEdit(
+      batchInput(
+        [
+          {
+            // no-op: data-start already "1", track already 0
+            element: el({ id: "a", tag: "video", domId: "a", start: 1, track: 0 }),
+            buildPatches: (original, target) =>
+              buildTimelineMoveTimingPatch(original, target, 1, 5, 0),
+          },
+          {
+            // real change: track 1 -> 2
+            element: el({ id: "b", tag: "video", domId: "b", start: 2, track: 1 }),
+            buildPatches: (original, target) =>
+              buildTimelineMoveTimingPatch(original, target, 2, 5, 2),
+          },
+        ],
+        writes,
+      ),
+    );
+
+    expect(writes).toHaveLength(1);
+    expect(writes[0]![0]).toBe("index.html");
+    expect(writes[0]![1]).toContain('id="b" class="clip" data-start="2" data-track-index="2"');
+  });
+
+  it("saves nothing when every member is a no-op", async () => {
+    stubReadFileContent(SOURCE);
+    const writes: Array<[string, string]> = [];
+
+    await persistTimelineBatchEdit(
+      batchInput(
+        [
+          {
+            element: el({ id: "a", tag: "video", domId: "a", start: 1, track: 0 }),
+            buildPatches: (original, target) =>
+              buildTimelineMoveTimingPatch(original, target, 1, 5, 0),
+          },
+        ],
+        writes,
+      ),
+    );
+
+    expect(writes).toHaveLength(0);
+  });
+});
+
 describe("deleteSelectedKeyframes", () => {
   it("coalesces all removals and reloads only after the last one", () => {
     usePlayerStore.setState({