refactor: unify doStuff/undoStuff via FILE_OPS config table

[jacaudi]

Summary

• Collapse the two near-identical doStuff (encrypt, 34 lines) and undoStuff (decrypt, 34 lines) functions in src/js/ui/file-ops.js into one processFileBatch(direction) driver.
• Introduce a FILE_OPS config table with enc and dec entries capturing the per-direction axes (worker call, buffer transform, filename transform, error message, status flag).
• Preserve doStuff and undoStuff as 1-line wrappers so file-import.js callers keep working unchanged.
• Add regression tests (public entry points still exist as top-level functions) and source-level tests (FILE_OPS shape + processFileBatch + filename transform round-trip).

Test plan

• docker run --rm -v $(pwd):/work -w /work node:22-alpine sh -c "node scripts/build.js && node --test app/tests/test-*.js" — 174/174 passing (166 baseline + 8 new)
• doStuff and undoStuff still declared as top-level functions in built app/index.html
• Filename round-trip: decSuffix(encSuffix(name)) === name for foo.txt, report.pdf, no-extension
• Manual round-trip in browser (encrypt → decrypt → verify contents byte-identical to original)

Out of scope

handleSharedFile (share path) is explicitly kept out of this refactor per the design — it has a different flow.

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