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.

scripts/show_calendar.sh

@@ -0,0 +1,5 @@
+#!/usr/bin/env bash
+
+export LESSCHARSET=utf-8
+TERM_WIDTH=$(tput cols)
+rem -m -cu+2 -b1 -gaa -q -@2 --hide-completed-todos -w"$TERM_WIDTH" | less -r +g --status-line -PM"Riga %lt/%L (%Pb\%)"