EVA-166: surface cron scheduling limitation in trigger config (B15)

[suzil]

Summary

• Adds an info banner in TriggerForm when the Cron trigger type is selected
• Banner explains that cron triggers fire while Eva is running, the scheduler checks every minute, and missed runs during downtime are not replayed
• No functional change — documentation addition only

Test plan

• Select Cron trigger type in a Trigger node detail panel — banner appears
• Switch to Manual trigger type — banner does not appear

Closes EVA-166

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