perf(ui/state): borrow &str key in compute_grouped_rows, drop N per-connection String clones

[obchain]

Closes #409

What

compute_grouped_rows is called every render frame when grouping is active. It bucketed connections into a HashMap<String, Vec<&Connection>>, cloning conn.process_name once per connection to produce an owned String key:

// before
let mut groups: HashMap<String, Vec<&Connection>> = HashMap::new();
for conn in connections {
    let key = conn
        .process_name
        .clone()                                     // one String clone per connection
        .unwrap_or_else(|| "<unknown>".to_string()); // one alloc for the fallback
    groups.entry(key).or_default().push(conn);
}

With N visible connections this produced N String allocations — one per connection — that existed only as map keys and were dropped after building the group stats. The connections: &'a [Connection] slice is already live for the full lifetime 'a, so the process name can be borrowed directly:

// after
let mut groups: HashMap<&'a str, Vec<&'a Connection>> = HashMap::new();
for conn in connections {
    let key = conn.process_name.as_deref().unwrap_or("<unknown>"); // zero alloc
    groups.entry(key).or_default().push(conn);
}

The downstream group_stats intermediate changes from Vec<(String, …)> to Vec<(&'a str, …)>. GroupedRow still stores process_name: String, so the name.clone() calls at row-push sites become name.to_owned() — the same allocation count there, but the N per-connection clones in the grouping loop are eliminated.

expanded_groups.contains(&name) (where name: String) becomes expanded_groups.contains(name) (where name: &str), valid via String: Borrow<str>.

Impact

Per grouped render: −N String allocations where N is the number of visible connections. With 100 connections across 10 groups, that is 100 clone + 100 heap-alloc + 100 drop cycles saved every frame.

Verification

cargo build --workspace            # clean
cargo test --workspace --lib       # 78 passed, 0 failed
cargo fmt --check                  # clean
cargo clippy --all-targets -- -D warnings  # clean