SFTP: Support IPv6 host addresses

[BrianLeishman]

Summary

The SFTP authority parser uses rsplit_once(':') to separate host and port, which breaks on IPv6 addresses containing multiple colons.

Current State

• File: src-tauri/src/locations/sftp/mod.rs:334
• parse_sftp_authority("user@::1") incorrectly parses as host=::, port=1
• parse_sftp_authority("user@[::1]:2222") would also fail

Proposed Changes

Support bracketed IPv6 literals per RFC 3986:

• sftp://user@[::1]/path → host=::1, port=22
• sftp://user@[::1]:2222/path → host=::1, port=2222

Parse logic:

1. If host_part starts with [, find matching ]
2. Extract IPv6 address from brackets
3. Check for :port after closing bracket

Acceptance Criteria

• sftp://user@[::1]/path parses correctly
• sftp://user@[::1]:2222/path parses correctly
• sftp://user@[fe80::1%25eth0]:22/path works (link-local with zone ID)
• Existing IPv4 and hostname parsing unchanged
• Unit tests cover all IPv6 formats