fix(contribution): sanitize amount input and a11y polish

app/components/ContributionAmountField.vue

@@ -58,6 +58,7 @@
           type="button"
           class="contribution-preset-chip"
           :class="{ active: numericAmount === preset.amount }"
+          :aria-pressed="numericAmount === preset.amount"
           @click="selectPreset(preset.amount)"
         >
           ${{ preset.amount }}
@@ -97,7 +98,11 @@ import {
 
 const props = defineProps({
   modelValue: { type: Number, required: true },
-  cadence: { type: String, required: true },
+  cadence: {
+    type: String,
+    required: true,
+    validator: (v) => v === 'monthly' || v === 'annual',
+  },
   allowCadenceChange: { type: Boolean, default: true },
   showSummary: { type: Boolean, default: true },
   summaryNote: { type: String, default: '' },
@@ -134,15 +139,14 @@ const onAmountInput = (event) => {
     return
   }
   const n = Number(raw)
-  emit('update:modelValue', Number.isFinite(n) ? n : 0)
+  const sanitized = Number.isFinite(n) ? Math.max(0, Math.trunc(n)) : 0
+  emit('update:modelValue', sanitized)
 }
 
 const selectPreset = (amount) => {
   emit('update:modelValue', amount)
 }
 
-// Annual→monthly snap: floor(annual/12)*12 keeps the amount cleanly divisible
-// before we divide, so toggling cadences never leaves fractional dollars.
 const onCadenceChange = (newCadence) => {
   if (newCadence === props.cadence) return
   const current = numericAmount.value