refactor: migrate remaining dialogs to Vue Transition, remove old keyframes

Convert Settings, Invoices, IdlePrompt, AppTrackingPrompt, and
AppDiscard dialogs from animate-modal-enter CSS class to proper
<Transition name="modal"> wrappers for enter/leave animations.
Remove unused animate-modal-enter and animate-dropdown-enter keyframes.

src/components/AppDiscardDialog.vue

@@ -0,0 +1,38 @@
+<script setup lang="ts">
+defineProps<{ show: boolean }>()
+defineEmits<{
+  cancel: []
+  discard: []
+}>()
+</script>
+
+<template>
+  <Transition name="modal">
+  <div
+    v-if="show"
+    class="fixed inset-0 bg-black/70 backdrop-blur-[4px] flex items-center justify-center p-4 z-[60]"
+    @click.self="$emit('cancel')"
+  >
+    <div class="bg-bg-surface border border-border-subtle rounded-lg shadow-[0_1px_3px_rgba(0,0,0,0.3)] w-full max-w-xs p-6">
+      <h2 class="text-[1.125rem] font-semibold font-[family-name:var(--font-heading)] text-text-primary mb-2">Unsaved Changes</h2>
+      <p class="text-[0.75rem] text-text-secondary mb-6">
+        You have unsaved changes. Do you want to discard them?
+      </p>
+      <div class="flex justify-end gap-3">
+        <button
+          @click="$emit('cancel')"
+          class="px-4 py-2 border border-border-subtle text-text-secondary rounded-lg hover:bg-bg-elevated transition-colors duration-150"
+        >
+          Keep Editing
+        </button>
+        <button
+          @click="$emit('discard')"
+          class="px-4 py-2 border border-status-error text-status-error font-medium rounded-lg hover:bg-status-error/10 transition-colors duration-150"
+        >
+          Discard
+        </button>
+      </div>
+    </div>
+  </div>
+  </Transition>
+</template>