feat(export): bundle multi-subnet export into a zip with a combined txt

[Mathious6]

What changed

• Exporting several subnets at once now writes one ZIP into the chosen folder: one <category>.txt per subnet plus an ALL_<qty>.txt concatenating every proxy line for easy bulk import.
• Single-subnet export keeps writing a plain TXT, and the AYCD JSON flow is untouched.
• Archive names follow the existing category grammar: shared tag stem first, else the shared country code, else XX / MIXED, then the subnet and proxy counts, and a short scope fingerprint so distinct selections never silently overwrite each other while re-exporting the same scope replaces its own archive.
• New archive.rs owns ZIP serialization (deflate, owner-only permissions inside and out); secure_file exposes the shared truncating 0600 opener; export planning stays pure and tested.
• README step 6 documents the bundled artifact.

Why

One downloadable artifact per export makes multi-subnet hand-off to bulk-import tools a single step instead of gathering many files.

Test plan

• cargo fmt --check, clippy -D warnings, cargo test --locked --lib (98 passed, incl. ZIP round-trip, overwrite, permissions, naming, scope-fingerprint cases)
• bun run check, bun run build
• Rebased on current main (frontend + rust dep groups) and re-validated both stacks
• Manual dev-instance check: multi-select export produces one ZIP with per-subnet TXTs and the combined ALL file