feat(viewer): add zoom controls to processing pdf viewer#63
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Summary
Closes #57.
Adds a zoom toolbar (−, %, +, Reset) to the processing PDF viewer (
scan_process.html, used in steps 1, 2, and 3). Range 0.5x – 3x, step 0.1. Step 1 persists its zoom level independently from steps 2/3, since reviewing page numbers and reviewing opinions benefit from different zooms.How it works
The toolbar lives at the top-left of the viewer panel and stays visible when you scroll, even horizontally at high zoom. Each PDF page is scaled visually as a whole, so detection boxes, redactions, and image overlays all stay aligned with the page automatically. Drawing a new redaction or detection, and dragging or resizing existing ones, works the same at any zoom level.
pdf.js rasterizes each page once into a canvas at a fixed resolution, so simply CSS-scaling that bitmap to a higher zoom looks really blurry above the original render scale (the same effect as zooming into a JPEG). To avoid this, when the zoom level changes the viewer re-rasterizes visible pages at the new resolution so text stays sharp. Pages outside the viewport are dropped and re-rendered fresh when scrolled back into view, which keeps memory bounded at high zoom on long documents. Rapid zoom clicks are debounced and any in-flight render is cancelled when zoom changes again, the same pattern pdf.js's own demo viewer uses.
The page header (PDF p.n + action buttons) also got a small layout tweak: at narrow widths (low zoom or small monitors) the buttons wrap below the page identifier instead of overflowing the page width.