feat(contacts): direct peer-to-peer connections via connection strings

[chanderlud]

Summary

Contacts can now be marked as "direct connections" with an Iroh endpoint address string. This lets peers connect directly without going through the relay server — useful for LAN setups or when both sides have public addresses.

What changed

• Contact model — Contact gains is_direct and direct_connection_string fields, persisted through the profiles JSON backend
• Session manager — when opening a session for a direct-contact, the manager parses the connection string into an EndpointAddr and calls endpoint.connect() directly instead of relaying through the discovery service
• Networking settings — a new "Your Connection Strings" section displays the local endpoint's addresses (direct addresses + relay URL) that can be shared with other users
• Contact forms — add-contact and edit-contact dialogs now show a Direct Connection toggle and a connection string text field

Design decisions

• Uses serde_json to serialize EndpointAddr (Iroh's type already implements Serialize/Deserialize) — no custom encoding needed
• The local endpoint address is stored in CoreState after bind and cleared on teardown to prevent stale info
• Backward compatible — existing contacts without direct fields load with is_direct: false and direct_connection_string: null

Test plan

• Rust unit and integration tests pass (53/53)
• Dart unit tests pass with updated mocks for the new
  odeAddr() method
• System tests should be run manually in WSL

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