fix(pdf-server): render page before O(numPages) annotation scans

[ochafik]

Problem

After the loading spinner clears, the viewer briefly shows an empty document — toolbar visible, page counter reads "1 of 1", canvas is a tiny blank white rectangle at default size — before the real page paints. ~400ms with a 15-page doc, scales linearly with page count.

Root cause

#506 added loadBaselineAnnotations() and buildFieldNameMap() between showViewer() and renderPage(). Both iterate every page calling getPage() + getAnnotations(). The canvas isn't sized until renderPage() runs, so the viewer sits empty for the duration of those scans.

Same pattern at the reload path.

Fix

Render first, scan after. Neither scan blocks the canvas — page.render() only needs canvasContext + viewport. renderAnnotationsForPage() reads annotationMap (graceful empty), AnnotationLayer.render() takes cachedFieldObjects (graceful null). A second renderPage() after the scans overlays baseline annotations and restored form values; for PDFs with neither, the canvas content is identical so there's no flicker.

Restores pre-#506 timing: page visible <50ms after showViewer().

Before/after (1706.03762, 15 pages)

Time	Before	After
~800ms	spinner	spinner
~1200ms	spinner	rendered
~1500ms	empty viewer, blank canvas	rendered
~1800ms	rendered	rendered

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commit: a490373