Add every(N) bind event and FZF_IDLE_TIME env var

- every(N) fires every N seconds (fractional, floored to 0.01s)
- Encoded as tui.Every with duration in Char as milliseconds, so
  every(1) and every(2) coexist as distinct keymap entries
- FZF_IDLE_TIME exposes whole seconds since the last user activity
  (keystroke or mouse event); pair with every() for idle-based
  patterns like auto-accept/auto-quit

Close #1211

src/tui/tui.go

@@ -229,6 +229,7 @@ const (
 	ClickHeader
 	ClickFooter
 	Multi
+	Every
 )
 
 func (t EventType) AsEvent() Event {
@@ -253,6 +254,10 @@ func (e Event) KeyName() string {
 		return me.Name()
 	}
 
+	if e.Type == Every {
+		return "every(" + strconv.FormatFloat(float64(e.Char)/1000, 'f', -1, 64) + ")"
+	}
+
 	if e.Type >= Invalid {
 		return ""
 	}