feat: add tmux popup commands for reminders and calendar

- Refactor `main.sh` to support `reminders` and `calendar` commands,
  displaying tmux popups with localized titles and current date/time.
- Add `show_reminders.sh` and `show_calendar.sh` scripts to render
  reminders and 2-week calendar views using `rem`.
- Update `tmux-remind.tmux` to bind `prefix + r` for reminders and
  `prefix + c` for calendar popups.
- Improve script robustness with `set -euo pipefail` and dynamic script
  directory resolution.

tmux-remind.tmux

@@ -2,11 +2,9 @@
 
 CURRENT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 
-# Carica script principale
-"$CURRENT_DIR/scripts/main.sh"
-
-# Esempio: binding di una key
-tmux bind-key -T prefix P run-shell "$CURRENT_DIR/scripts/main.sh"
-# bind-key -T prefix r display-popup -T "Impegni di oggi…" -w 50% -h 50% -x C -y C -E -- "$CURRENT_DIR/scripts/main.sh"
+# "$CURRENT_DIR/scripts/main.sh"
 
+# tmux bind-key -T prefix r display-popup -T "Impegni di oggi…" -w 50% -h 50% -x C -y C -E -- "$CURRENT_DIR/scripts/main.sh"
 
+tmux bind-key -T prefix r run-shell "$CURRENT_DIR/scripts/main.sh reminders"
+tmux bind-key -T prefix c run-shell "$CURRENT_DIR/scripts/main.sh calendar"