perf(ui/overview): eliminate String allocs in connections_title and per-row connector span

[obchain]

Problem

Two allocation patterns in src/ui/tabs/overview.rs:

1. connections_title — two allocations per frame for static content

let mut base = if ui_state.show_historic {
    "Active + Historic Connections".to_string()   // alloc
} else {
    "Active Connections".to_string()              // alloc
};
if grouped {
    base.push_str(" · Grouped by Process");        // possible realloc
}
// …then:
Span::styled(format!(" {base}"), …)              // alloc

The four possible combinations of (show_historic, grouped) each produce a distinct static string. A single match covers all four without allocating:

let base: &'static str = match (ui_state.show_historic, grouped) {
    (false, false) => " Active Connections",
    (false, true)  => " Active Connections · Grouped by Process",
    (true,  false) => " Active + Historic Connections",
    (true,  true)  => " Active + Historic Connections · Grouped by Process",
};
Span::styled(base, …)   // zero alloc: &'static str → Cow::Borrowed

The leading space is embedded in each static string, so the format!(" {base}") call is also eliminated. Saves 2+ heap allocations per frame.

2. Per-row connector — connector.to_string() in the expanded-group render loop

let connector: &'static str = if is_last { "  └─ " } else { "  ├─ " };
// …
Span::styled(connector.to_string(), theme::fg(theme::muted()))  // alloc per row

connector is already &'static str. Span::styled accepts T: Into<Cow<'static, str>>, which &'static str satisfies directly — to_string() is unnecessary.

Span::styled(connector, theme::fg(theme::muted()))  // zero alloc

Saves 1 heap allocation per expanded connection per frame.

Impact

• connections_title: −2 to −3 allocations every frame regardless of what is visible.
• Connector: −1 allocation per visible expanded-group connection per frame.