fix: clarify failed host-only server connections

[w3lld1]

Summary

• I added a clear Jellyfin default-port hint to the connection failure dialog when the submitted address contains only a hostname or IP address.
• I keep the existing error unchanged for addresses that already include a scheme, port, path, query, or fragment.
• I added focused coverage for hostnames, IPv4, IPv6, and explicit connection details.

Validation

• node --test src/web/server-url.test.js
• node --check src/web/server-url.js
• node --check src/web/overlay.js
• cargo fmt --manifest-path src/Cargo.toml --all -- --check
• cargo test --manifest-path src/Cargo.toml -p jfn-jellyfin (29 passed)
• git diff --check origin/main...HEAD

I could not complete the full jfn-cef test build locally because the environment lacks mpv/client.h, and unpacking the CEF binary exceeded the available local disk quota. The focused UI logic checks above pass; I am leaving the full platform build to CI.

Fixes #402