feat: add flip PDF operation

[chrisriv10]

Which issue does this close?

Closes #199

What changed

Adds a new Flip PDF operation (src/operations/flip-pdf/) that mirrors every page of a PDF:

• Horizontal — mirrors left ↔ right
• Vertical — mirrors top ↔ bottom
• Both — mirrors both axes

Uses pdf-lib with supported xScale/yScale negative scaling + translation (not the unsupported transformationMatrix). Preserves original page dimensions, supports multi-page PDFs, fully client-side with no new dependencies.

Acceptance criteria

• Pages are mirrored on the chosen axis in the exported PDF
• Works on multi-page PDFs
• Runs fully offline
• Follows the plugin pattern: self-contained src/operations/flip-pdf/{meta.js,index.jsx,helpers.js}
• Reuses shared components (Dropzone, useJob, Progress, Note, DownloadButton, usePdfPageCount)
• Auto-discovered via registry globs (no central wiring)
• Order: 6 (appears right after Rotate PDF)

Checklist

• Runs fully offline: no network calls, no CDNs, no external assets (strict CSP connect-src 'self')
• Follows the plugin pattern and reuses shared components where relevant
• npm run build passes locally

Proof

• npm run lint ✅ (0 errors, 19 pre-existing warnings)
• npm run format:check ✅
• npm run build ✅ (559 modules, 51 prerendered pages)
• npm run test ✅ (40 tests in 4 files)

Manual verification with asymmetric test PDF (400×300, 2 pages):

• Horizontal: LEFT ↔ RIGHT swapped, TOP/BOTTOM unchanged
• Vertical: TOP ↔ BOTTOM swapped, LEFT/RIGHT unchanged
• Both: both axes flipped
• Page count preserved (2 → 2), dimensions unchanged (400×300)

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