Add replay button to timer completion popup

[sirgeb27]

Summary

Adds a replay button (SELECT/middle) to the "Time's Up!" popup, which
restarts the timer with its original duration. The existing UP=snooze
(1 min) and DOWN=dismiss buttons are unchanged.

Use case

When using timers for repetitive actions — holding stretches, power
hours, interval training — the current flow requires two button presses
to restart: dismiss the popup, then navigate back to the timer and
press play. The replay button reduces this to one press from the
completion screen.

Changes

• src/popup_window.c — load/destroy replay_icon; assign to
  BUTTON_ID_SELECT on the action bar
• src/popup_window.h — updated PopupWindowSelectClick typedef to
  pass CountdownTimer* (consistent with PopupWindowUpClick)
• src/main.c — popup_window_replay_timer_callback() restarts the
  timer using its original duration via countdown_timer_get_duration()
• package.json — registers IMAGE_REPLAY resource
• resources/images/action_bar_icon_replay.png — circular arrow icon

Notes

The SELECT button callback slot was already scaffolded in
PopupWindowCallbacks with a null-check but unused. This PR fills it.

Aware of #7 (popup layout overlap on taller screens) — that fix is
complementary and doesn't conflict with this change.