[Enhancement] Before/after preview for image and PDF compression #6

Priority: P2
Parent roadmap: #1

Problem

A file being under the website upload limit says nothing about whether it still looks good. Tuck should offer a lightweight optional visual review of compressed images and PDFs without imposing user-visible quality presets.

UX

• After compression (or from Jobs/history), open a native comparison of source vs generated copy: side-by-side and slider/split view where appropriate; sync zoom/pan, fit to window and show pixel dimensions, original/result bytes and reduction.
• For multi-page PDFs show synchronized page navigation, text/page rendering checks and a sampled side-by-side comparison, including page count. Flag PDF structure differences such as lost selectable text separately from appearance.
• Show basic output diagnostics: format, color profile, transparency/alpha, dimensions, page count and actual strict byte cap achieved.
• Provide Reveal in Finder, Keep result and optionally Discard generated copy with clear confirmation and identity check. Never overwrite or delete input.
• Keep the default one-click workflow unchanged; comparison is opt-in, not a mandatory dialog and not an encoder-quality setting.

Acceptance criteria

• Image and PDF previews agree with opened source/output files (color, rotation, transparency, page order).
• Results below a target remain under that exact cap after preview; no re-encode on preview.
• Huge images and PDFs load incrementally/cached/throttled without exhausting 8 GB RAM.
• Invalid, missing, encrypted or unsupported preview files get useful error messages.
• Keyboard navigation and VoiceOver labels supported.

Tests

Use representative photographs, transparent PNGs, documents with mixed vector/raster pages, PDFs with selectable text, and large/multipage files. Validate source hashes and correct display at different zoom levels. Depends on stable compression and may integrate with #4 history/jobs.