fix(macos): restore titlebar double-click with hidden tab strip

[jxdones]

Closes #1786.

Summary

• Route clicks in the hidden single-tab chrome through the existing window drag and double-click handler.
• Use the default cursor over the reserved macOS chrome area.
• Preserve the existing behavior on Linux and Windows.

Testing

• cargo fmt --all -- --check
• cargo test -p rioterm (183 passed)

[raphamorim]

Reviewed against the padding and input routing paths. The core is right: with hide-if-single on macOS, padding_top_from_config reserves ISLAND_HEIGHT whenever Tab navigation is enabled (the render-from-top early return is non-macOS only), so the band really is dead chrome and routing its clicks through on_chrome_press restores drag and double-click maximize with the same state machine the visible island uses. The cursor gate also matches the padding predicate exactly, and NativeTab/Plain modes are untouched since is_enabled() is Tab-only.

Two small requests inline, and one note: the UCRT64 CI failure is not yours. Our new PR smoke lane built the whole workspace in debug, and the cdylib targets blow the PE export-ordinal limit on windows-gnu. Fixed on main; a re-run should come back green.

[raphamorim]

Right-clicks here fall through to the grid, which starts below the band, so the click acts on the first terminal row while the pointer is over chrome. The visible-island path consumes right-clicks in its chrome regions (the x_in_tabs < 0 and past-last-tab arms return true without an action). Consuming them here too would keep the hidden band consistent with the visible one:

#[cfg(target_os = "macos")]
{
    if !is_right_click {
        self.on_chrome_press(window, chrome_press);
    }
    return true;
}

[raphamorim]

This predicate now exists in three places that must agree: here, the new branch in screen/mod.rs, and padding_top_from_config, which is the drift island_visible's own doc comment warns about. A helper on Navigation would pin them together, something like:

/// Whether the top band is reserved as window chrome this frame,
/// painted island or not. Must agree with padding_top_from_config.
pub fn chrome_band_reserved(&self, num_tabs: usize) -> bool {
    if cfg!(target_os = "macos") {
        self.is_enabled()
    } else {
        self.island_visible(num_tabs)
    }
}

Then this line and the screen branch both call it, and the cfg in screen/mod.rs collapses into the same expression.