feat: add alternate PDF operation

[Mithurn]

Closes #200

What changed

• Adds an Alternate PDFs operation that interleaves pages from two or more PDFs (A1, B1, A2, B2, …).
• Adds a Reverse the second file option for backs scanned last-page-first. Leftover pages from a longer file are appended.
• Self-contained plugin at src/operations/alternate-pdf/, plus a shuffle glyph in Icon.jsx. No new dependencies.

Acceptance criteria (from the issue)

• Pages from the inputs are interleaved in order into one PDF
• A reverse-second-file option works for double-sided scans
• Runs fully offline

Proof

9 unit tests and a Playwright e2e spec; npm run build and npm run lint pass.

Output page order. Green pages came from the first file, blue from the second, so the alternating colours show the interleave and 1-6 shows the reverse option correcting the backs.

Network tab on the production build: downloading the result makes zero requests.

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[mithun-srinivas]

Really well-rounded PR @Mithurn, thank you. The interleave plan handles two or more files and appends leftovers from the longer one, the reverse-second option covers duplex scans, and using copyPages keeps the pages as vectors so text stays selectable. Shipping unit tests plus an e2e spec and the zero-network proof is exactly the standard we love to see.

Meets every criterion on #200 and all checks are green. Merging now.

One tiny thing for next time, no action needed here: the Icon.jsx change also dropped the unused iconNames export, which is unrelated to this feature. It is dead code so no harm done, just easier to review when unrelated cleanups ride in their own PR. Thanks again!