feat(calendar): extend calendar view to show next 4 weeks

Update both the tmux popup title and the `rem` command in
`show_calendar.sh` to display events for the next 4 weeks instead of 2
weeks.

scripts/main.sh

@@ -33,7 +33,7 @@ function show_reminders {
 function show_calendar {
     local datetime
     datetime=$(get_datetime)
-    tmux display-popup -T "#[align=centre]Next 2 Weeks - $datetime" -w 99% -h 99% -x C -y C -E -- "$CURRENT_DIR/show_calendar.sh"
+    tmux display-popup -T "#[align=centre]Next 4 Weeks - $datetime" -w 99% -h 99% -x C -y C -E -- "$CURRENT_DIR/show_calendar.sh"
 }
 
 function main {