Tighten footer keybind packing slack

[Antoinegtir]

Scope

The footer keybind packing in crates/fl-tui/src/render.rs (pack_footer_binds) computes available width using unicode-width style logic, but for plain ASCII single-character keybinds it sometimes leaves >2 chars of trailing whitespace before truncating — meaning an extra binding could have fit.

What to do

• Add a test (snapshot or unit) that checks: at width N where exactly K bindings fit with the current separator, no binding is dropped.
• Tighten the packing loop so the trailing slack is at most one separator wide.

Where

crates/fl-tui/src/render.rs — search for pack_footer_binds and FOOTER_BINDS.

Acceptance

• New test covers the "almost-fits" case.
• Manual check at terminal widths 60, 80, 100, 140 shows no wasted slack on the right edge.