perf(ui/overview): use static strings in title and connector spans

[obchain]

Closes #411

What

Two allocation patterns eliminated in src/ui/tabs/overview.rs. All 78 lib tests pass unchanged.

1. connections_title — match on booleans instead of building a String

// before: 2–3 allocations per frame
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");       // may realloc
}
Span::styled(format!(" {base}"), …)             // alloc

// after: 0 allocations per frame
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, …)  // &'static str → Cow::Borrowed, no heap

The leading space is baked into each static string, eliminating the format!(" {base}") call entirely. Saves 2–3 allocations every frame.

2. Connector span in expanded-group rows

// before: 1 alloc per expanded connection per frame
Span::styled(connector.to_string(), theme::fg(theme::muted()))

// after: 0 allocs
Span::styled(connector, theme::fg(theme::muted()))

connector is &'static str (either " └─ " or " ├─ "). Span::styled accepts T: Into<Cow<'static, str>>, which &'static str satisfies directly. Saves 1 allocation per visible expanded-group connection per 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