Open PCAP files from Linux and Windows file managers

[SR0725]

This connects file-manager "Open with" actions to Sniffnet's existing PCAP import flow on Linux and Windows.

Changes:

• accept a PCAP path as a positional CLI argument and start offline analysis
• register application/vnd.tcpdump.pcap in the Linux desktop entry
• register .pcap, .pcapng, and .cap in the Windows MSI
• keep the existing --adapter startup path and reject conflicting capture sources

macOS is not included in this change. The current app bundle starts through a shell wrapper, while Launch Services delivers an open-document event instead of a file argument. Supporting that path needs a native event bridge rather than another file-association entry.

Verification:

• cargo test — 232 passed
• cargo clippy -- -D warnings
• cargo fmt --all -- --check
• cargo run -- --help
• xmllint --noout resources/packaging/windows/setup.wxs

I could not build the Windows MSI locally from macOS, so the Windows packaging job remains the platform-specific validation for that part.

AI assistance: I used an AI coding assistant while investigating and implementing this change. I reviewed the diff and can explain each change.

Related to #1288.

[GyulyVGC]

Hey @SR0725 thank you!

Nice to see that implementing support for this is quite simple.
Ideally I'd like to implement support for all the first-class supported platforms (including macOS) all at once, but this can be a good starting point.