feat: add optional logical-size screenshot exports

[tusharmalpani20]

Add [output] logical_size = true to optionally downscale Copy/Save output on scaled monitors. For example, a 600-pixel-wide capture at 2x exports at 300 pixels. Native-resolution output remains the default.

Resize the completed image after rendering annotations, backdrops, and redactions. Apply the setting to editor exports and fullscreen quick output. Preserve the original monitor scale for stitched captures so logical-size exports also work after reopening their working
documents.

Editing coordinates, source pixels, OCR, and pins retain their existing behavior. Fractional dimensions round to the nearest pixel, with a one-pixel minimum.

Fixes #119.

Validation:

• QT_QPA_PLATFORMTHEME=generic make check passed.
• git diff --check passed.
• Added coverage for defaults, invalid settings/scales, fractional scaling, transparency, and tiny images.
• Verified matching Copy/Save pixels with annotations and unchanged working sources.
• Verified immediate and reopened scrolling exports at 1.5x and 2x.
• Verified downscaling preserves redaction’s removal of source spatial information.