Release 0.8.0

[mwmdev]

Prepares slashmail 0.8.0.

Includes the unreleased commits on main since v0.7.0 (read --uid, JSON thread IDs and flags, JSON draft receipts) and the audit remediation. See CHANGELOG.md for the full list.

Breaking changes:

• move takes its destination as --dest; --to is always the recipient filter.
• Plaintext IMAP is refused for non-loopback hosts.
• Export file names percent-encode folder names.
• -f/--folder cannot be combined with --all-folders.
• Minimum Rust version is 1.88 (mailparse 0.17).

Verification:

• Linux: fmt, clippy (all targets and features, warnings denied), unit, bin, GreenMail integration, and scripted CLI tests.
• Rust 1.88: cargo test --locked.
• Windows: MinGW cross-build, with lib, bin, and scripted CLI tests run under Wine.
• cargo package, cargo publish --dry-run, and cargo deny check.
• Website release section previewed headlessly.