feat: enhance UI and functionality for price rules, quotes, and suppliers

- Updated PriceRulesPage.vue to replace "Neu" button with a more intuitive "+" button for creating new rules.
- Enhanced QuotesPage.vue with improved handling of quote lines, including read-only states, unique line validation, and better formatting for monetary values.
- Added accordion sections for better organization of quote details and improved user experience.
- Updated SuppliersPage.vue to replace "Neu" button with a "+" button for adding new suppliers.
- Introduced new database migrations to add price category and default sales price fields to activities, and to include 'invoice_created' status in quotes.

web-frontend/src/views/CashDiscountTermsPage.vue

@@ -82,7 +82,7 @@ watch(() => liveUpdateState.revision, load);
   <PageHeader title="Skonto-Regeln" description="Zahlungsbedingungen für Kunden, Lieferanten und Belege." />
   <div class="workspace-split">
     <section class="panel list-panel">
-      <div class="section-title"><h2>Regeln</h2><button type="button" @click="createNew">Neu</button></div>
+      <div class="section-title"><h2>Regeln</h2><button type="button" class="add-button" title="Neue Regel" @click="createNew">+</button></div>
       <button v-for="term in terms" :key="term.id" type="button" class="list-row" :class="{ selected: selectedId === term.id }" @click="select(term)">
         <strong>{{ term.code }}</strong>
         <span>{{ term.name }}</span>