archive: volume-aware extract/compress + friendlier errors

[lovehunter9]

Summary

A bundle of multi-volume archive fixes plus a CORS hole that blocked the iOS app from sending X-Archive-Password.

• Extract / Entries / Entry: any volume the user clicks (.002, .003, ...) is now rewritten to .001 server-side, so the request resolves regardless of which volume they picked. A directory pre-scan rejects requests where an intermediate .NNN is missing (e.g. user deleted .002); a fallback map (mapExtractErr) translates 7z's raw exit status 2 into an actionable message when the missing piece is the last volume and can't be detected up front.
• Compress rename/skip/overwrite are no longer fooled by partial volume sets: scanning any <base>.NNN numeric sibling prevents the rename path from picking a name whose .001 clashes, the skip path from missing existing .002, and the overwrite path from leaving stray volumes behind. Basenames are also gated against POSIX NAME_MAX (255) before the task runs, so over-long names fail fast rather than blowing up multi-volume tmp paths mid-write.
• CORS Allow-Headers gains x-archive-password so the FE can send the password header across origins (capacitor / app webview).
• 7z Classify learns three more substring patterns commonly seen on missing-volume failures, so those errors land on ErrVolumeMissing instead of the generic exit status 2 we kept seeing.

Behavior

• Same single-volume archives behave exactly as before.
• Errors users see for partial volume sets are now: archive volume missing: foo.zip.002 (preflight) or archive volume missing or incomplete; please ensure all parts are present (task fallback) instead of exit status 2.
• Over-long compression names are rejected with archive name too long: N bytes (max ...) rather than failing inside 7z.

Test plan

• Right-click .001, .002, .003 of a 4-volume zip → all three trigger extract identically.
• Delete .002; right-click any volume → 400 archive volume missing: ...zip.002 (preflight catches it).
• Delete the last volume; right-click any → task fails with the friendly message.
• Compress test.zip to a directory containing test.zip.001..004 (or only .002) with conflict=rename → output goes to test (1).zip.001.. and the original volumes stay intact.
• Compress with conflict=skip on a directory holding any <base>.NNN → task returns immediately, no work done.
• Compress with conflict=overwrite → all existing <base>.NNN are backed up to .bak.<taskid> and restored on cancel/failure.
• iOS LarePass (Origin: capacitor://localhost) sending X-Archive-Password no longer trips CORS.

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