Fix "in N minutes" parsing and VHS tape errors

- Strip "in " prefix before passing to date -d (was always broken)
- Fix quickstart tape: -B3 → -B4 for resume grep (Job: is 4 lines back)
- Fix notifications tape: -f ~/backup.sh → -c (file doesn't exist)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

demo/notifications.tape

@@ -0,0 +1,62 @@
+# systab — Notifications
+# Shows status-aware desktop and email notifications.
+
+Output demo/notifications.gif
+Set Shell bash
+Set Width 1200
+Set Height 600
+Set FontSize 16
+
+Set TypingSpeed 50ms
+
+Sleep 1s
+
+# Desktop notification — success case
+Type "# Desktop notification on success/failure"
+Enter
+Sleep 1s
+Type "systab -t 'in 1 minute' -c 'echo success' -i"
+Sleep 500ms
+Enter
+Sleep 2s
+
+# Desktop notification — failure case
+Type "systab -t 'in 1 minute' -c 'exit 1' -i"
+Sleep 500ms
+Enter
+Sleep 2s
+
+# Email notification
+Type "# Email notification via sendmail"
+Enter
+Sleep 1s
+Type "systab -t 'in 1 minute' -c 'echo test' -m user@example.com"
+Sleep 500ms
+Enter
+Sleep 2s
+
+# Both notifications
+Type "# Both desktop and email"
+Enter
+Sleep 1s
+Type "systab -t 'every day at 9am' -c '/home/user/backup.sh' -i -m admin@example.com"
+Sleep 500ms
+Enter
+Sleep 2s
+
+# Show the generated service file to see ExecStopPost lines
+Type "# Inspect generated service file"
+Enter
+Sleep 1s
+Type "cat ~/.config/systemd/user/systab_*.service | head -20"
+Sleep 500ms
+Enter
+Sleep 4s
+
+# Check status
+Type "systab -S"
+Sleep 500ms
+Enter
+Sleep 3s
+
+Sleep 2s