fix(key_codes): add Function key arm so macOS fn can be bound to a chord (#950)

key_from_str() had no arm for "Function", so it fell through to
None. Since build_chord propagates that as a hard Err via ?, binding
any chord containing fn made build_chord_bindings fail entirely —
HotkeyMonitor was never spawned, silently killing both push-to-talk
and toggle-to-talk until the chord was reverted.

Every other layer (keytap's macOS key tap, Key::Function itself, the
frontend's canonicalKeyFromEvent/displayLabelForKey) already handles
fn — only this string-to-Key bridge was missing the arm.

Fixes #941

tauri/src-tauri/src/key_codes.rs

@@ -30,6 +30,7 @@ pub fn key_from_str(name: &str) -> Option<Key> {
         "ShiftLeft" => Key::ShiftLeft,
         "ShiftRight" => Key::ShiftRight,
         "CapsLock" => Key::CapsLock,
+        "Function" => Key::Function,
 
         // Whitespace / navigation
         "Space" => Key::Space,