fix: centralize zoom state and guard auto-fit during pinch

[vitormattos]

Problem

The component maintained two separate scale values — visualScale (for intermediate wheel/pinch steps) and scale (committed value) — that could diverge and cause elements to be positioned or measured with the wrong scale factor. Additionally, onViewportScroll would trigger an auto-fit recalculation while a pinch gesture was still in progress, creating a race condition between user zoom and auto-fit.

Changes

refactor: remove zoom utility module

• Deletes src/utils/zoom.ts and tests/utils/zoom.spec.ts.
• applyScaleToDocs is now fully absorbed by syncScaleState inside PDFElements, which is the single authoritative place that updates pagesScale.

fix: centralize zoom state and guard auto-fit during pinch

• pendingZoomScale: accumulates fast scroll-wheel ticks between RAF frames so scale always converges correctly under rapid input.
• syncScaleState(): single method that normalises, clamps, updates pagesScale on every document, and invalidates the measurement cache. All zoom paths (wheel, pinch, adjustZoomToFit, external scale prop mutation) now funnel through it.
• isSyncingScale flag: prevents the scale watcher from re-entering syncScaleState when the method itself writes back to this.scale.
• onViewportScroll now checks !isPinching before triggering auto-fit on resize, preventing concurrent auto-fit from fighting an in-progress pinch gesture.
• adjustZoomToFit accepts force = false so callers with autoFitZoom disabled can still trigger a one-shot fit.

test: add regression tests for zoom state sync and touch resize

Regression tests that prove the bugs exist on the old code and pass on the new code:

Scenario	Type
Wheel zoom accumulates and pagesScale stays in sync	regression
Direct scale prop mutation triggers syncScaleState via watcher	regression
Out-of-range scale value is clamped to supported bounds	regression
Auto-fit runs on touch devices when container width changes	regression
Auto-fit suppressed while isDraggingElement	guard
Auto-fit suppressed while isPinching; resumes after pinch ends	regression
Rotation-like resize (portrait → landscape) recomputes optimal scale	regression
Width unchanged → auto-fit NOT triggered (negative control)	guard
Forced one-shot fit works even when autoFitZoom prop is disabled	feature