[unjica]

Makes the NO_EVENTS scope explicit in AlertRuleEditor (radio: All events in the project / A specific event).

• All events: sends no eventName (original project-wide no-events behaviour), even if a name was typed earlier.
• A specific event: non-empty name required; Save disabled and validation error otherwise. Name is trimmed.
• Existing rules without eventName (absent or null) open as All events and round-trip unchanged; rules with a name open as A specific event.
• New heartbeat conditions default to A specific event.
• Fixes the UI/API inconsistency where an empty name field silently saved a watch-all rule. API unchanged (already rejects empty/whitespace and accepts omitted/null; covered by existing API tests).

No migration, no API change. Tests: dashboard vitest 373/373 (7 new scope tests, 1 updated).