fix(studio): add preview zoom controls (#761)

* fix(studio): add smooth preview zoom with pinch/Ctrl+scroll

- Scale iframe content from inside (contentDocument.documentElement) instead
  of scaling the parent div, avoiding compositor re-rasterization on every
  zoom frame — critical for smooth zoom on high-refresh displays (240Hz)
- Document-level capture-phase wheel handler bypasses the DomEditOverlay
- Center-based zoom (no pan drift from pointer-anchored formulas)
- Transient HUD shows zoom % briefly, no persistent UI controls
- Double-click preview area to reset zoom to fit
- Drag-to-pan when zoomed past 100%
- Momentum scroll suppression after pinch gesture (400ms cooldown)
- Delta clamping (MAX_DELTA=10) prevents overshooting on fast gestures
- toDomPrecision rounds transform values to 4 decimals (matches tldraw)
- Zoom state persisted to localStorage with 200ms debounce
- Exposes --preview-zoom CSS custom property for overlay coordinate mapping
- Fix infinite render loop in NLELayout (onIframeRef → refreshPreviewDocumentVersion)

* fix(studio): use CSS zoom instead of transform scale for preview zoom

CSS transform: scale() on a div containing an iframe causes compositor
cross-layer sync issues that produce visible frame tearing on high-refresh
displays (240Hz ProMotion). CSS zoom property changes the actual rendered
size without compositor layer synchronization, eliminating the jumping.

- Replace transform: scale(Z) with zoom: Z on the stage div
- Keep transform: translate() for panning (compositor-friendly, no iframe)
- Overlays work correctly since getBoundingClientRect() includes zoom
- Remove will-change, transition hacks, pointer-events toggles

* fix(ci): use apt-get for ffmpeg in preview-regression workflow

The FedericoCarboni/setup-ffmpeg action downloads from an external URL
that has been persistently unreachable, causing CI failures. Switch to
apt-get install which uses Ubuntu's package repos (same as ci.yml and
player-perf.yml).

* fix(studio): clear zoom timers on NLEPreview unmount

settleTimerRef, hudTimerRef, and retiringTimerRef could fire after
component unmount. Add cleanup effect to prevent stale callbacks.

* feat(studio): persist sidebar, timeline, and playback speed across reloads

Wire up studioUiPreferences for the three remaining UI states requested
in #752: left sidebar collapsed, timeline visibility, and playback rate.
All three now survive page reloads using the same localStorage key as
preview zoom.

packages/studio/src/utils/studioUiPreferences.ts

@@ -0,0 +1,84 @@
+export interface StoredPreviewZoomState {
+  zoomPercent: number;
+  panX: number;
+  panY: number;
+}
+
+export interface StudioUiPreferences {
+  leftCollapsed?: boolean;
+  timelineVisible?: boolean;
+  playbackRate?: number;
+  previewZoom?: StoredPreviewZoomState;
+}
+
+const STUDIO_UI_PREFERENCES_KEY = "hf-studio-ui-preferences";
+
+function isRecord(value: unknown): value is Record<string, unknown> {
+  return typeof value === "object" && value !== null;
+}
+
+function getBrowserStorage(): Storage | null {
+  if (typeof window === "undefined") return null;
+  try {
+    return window.localStorage;
+  } catch {
+    return null;
+  }
+}
+
+function readStorage(storage: Storage | null): StudioUiPreferences {
+  if (!storage) return {};
+  try {
+    const raw = storage.getItem(STUDIO_UI_PREFERENCES_KEY);
+    if (!raw) return {};
+    const parsed: unknown = JSON.parse(raw);
+    if (!isRecord(parsed)) return {};
+
+    const preferences: StudioUiPreferences = {};
+    if (typeof parsed.leftCollapsed === "boolean") {
+      preferences.leftCollapsed = parsed.leftCollapsed;
+    }
+    if (typeof parsed.timelineVisible === "boolean") {
+      preferences.timelineVisible = parsed.timelineVisible;
+    }
+    if (typeof parsed.playbackRate === "number" && Number.isFinite(parsed.playbackRate)) {
+      preferences.playbackRate = parsed.playbackRate;
+    }
+    if (isRecord(parsed.previewZoom)) {
+      const { zoomPercent, panX, panY } = parsed.previewZoom;
+      if (
+        typeof zoomPercent === "number" &&
+        Number.isFinite(zoomPercent) &&
+        typeof panX === "number" &&
+        Number.isFinite(panX) &&
+        typeof panY === "number" &&
+        Number.isFinite(panY)
+      ) {
+        preferences.previewZoom = { zoomPercent, panX, panY };
+      }
+    }
+    return preferences;
+  } catch {
+    return {};
+  }
+}
+
+export function readStudioUiPreferences(storage: Storage | null = getBrowserStorage()) {
+  return readStorage(storage);
+}
+
+export function writeStudioUiPreferences(
+  patch: StudioUiPreferences,
+  storage: Storage | null = getBrowserStorage(),
+) {
+  if (!storage) return;
+  try {
+    const next = {
+      ...readStorage(storage),
+      ...patch,
+    };
+    storage.setItem(STUDIO_UI_PREFERENCES_KEY, JSON.stringify(next));
+  } catch {
+    /* localStorage may be unavailable or full */
+  }
+}