ui(alerts): applyFireModeChange — clear mode-specific fields on toggle

Prevents stale COUNT_IN_WINDOW threshold/windowSeconds from surviving
PER_EXCHANGE save (would trip the Task 3.3 server-side validator).
Also forces reNotifyMinutes=0 and forDurationSeconds=0 when switching to
PER_EXCHANGE.

Turns green: form-state.test.ts#applyFireModeChange (3 tests).

ui/src/pages/Alerts/RuleEditor/condition-forms/ExchangeMatchForm.tsx

@@ -1,6 +1,7 @@
 import { FormField, Input, Select } from '@cameleer/design-system';
 import type { FormState } from '../form-state';
-import { EXCHANGE_FIRE_MODE_OPTIONS } from '../../enums';
+import { applyFireModeChange } from '../form-state';
+import { EXCHANGE_FIRE_MODE_OPTIONS, type ExchangeFireMode } from '../../enums';
 
 // ExchangeFilter.status is typed as `String` on the backend (no @Schema
 // allowableValues yet) so options stay hand-typed. Follow-up: annotate the
@@ -23,7 +24,7 @@ export function ExchangeMatchForm({ form, setForm }: { form: FormState; setForm:
       <FormField label="Fire mode">
         <Select
           value={(c.fireMode as string) ?? 'PER_EXCHANGE'}
-          onChange={(e) => patch({ fireMode: e.target.value })}
+          onChange={(e) => setForm(applyFireModeChange(form, e.target.value as ExchangeFireMode))}
           options={EXCHANGE_FIRE_MODE_OPTIONS}
         />
       </FormField>

ui/src/pages/Alerts/RuleEditor/form-state.ts

@@ -3,7 +3,7 @@ import type {
   AlertRuleResponse,
   AlertCondition,
 } from '../../../api/queries/alertRules';
-import type { ConditionKind, Severity, TargetKind } from '../enums';
+import type { ConditionKind, ExchangeFireMode, Severity, TargetKind } from '../enums';
 
 export type WizardStep = 'scope' | 'condition' | 'trigger' | 'notify' | 'review';
 export const WIZARD_STEPS: WizardStep[] = ['scope', 'condition', 'trigger', 'notify', 'review'];
@@ -137,6 +137,38 @@ export function toRequest(f: FormState): AlertRuleRequest {
   } as AlertRuleRequest;
 }
 
+/**
+ * Pure helper for the ExchangeMatchForm's Fire-mode <Select>. Guarantees state
+ * hygiene across toggles:
+ *
+ *   - Switching to PER_EXCHANGE clears COUNT_IN_WINDOW-only condition fields
+ *     (threshold, windowSeconds) AND forces top-level reNotifyMinutes = 0
+ *     and forDurationSeconds = 0 — PER_EXCHANGE is exactly-once-per-exchange,
+ *     so re-notify cadence and hold-duration are meaningless and must not leak
+ *     into toRequest().
+ *   - Switching back to COUNT_IN_WINDOW resets threshold/windowSeconds to 0
+ *     (never restoring stale values from the previous COUNT_IN_WINDOW session).
+ *
+ * No-op for non-EXCHANGE_MATCH conditions. Returns a new form object.
+ */
+export function applyFireModeChange(form: FormState, newMode: ExchangeFireMode): FormState {
+  const c = form.condition as Record<string, unknown>;
+  if (c.kind !== 'EXCHANGE_MATCH') return form;
+  const base: FormState = {
+    ...form,
+    condition: {
+      ...c,
+      fireMode: newMode,
+      threshold: 0,
+      windowSeconds: 0,
+    } as FormState['condition'],
+  };
+  if (newMode === 'PER_EXCHANGE') {
+    return { ...base, reNotifyMinutes: 0, forDurationSeconds: 0 };
+  }
+  return base;
+}
+
 export function validateStep(step: WizardStep, f: FormState): string[] {
   const errs: string[] = [];
   if (step === 'scope') {