feat(hours): install-wide default rate + inline missing-rate CTA

Hour-entry saves hard-failed with an English-only error when a customer
had no rate, and the standalone hours page showed a disabled rate field
that looked set. Add a global business_profile default_hourly_rate_minor
(migration 113) as the last link in the rate chain
(entry override → customer → install default), so saves succeed with the
global rate. When no rate resolves anywhere, replace the save-time error
with a read-only resolved-rate display + a CTA to set a customer or
install-wide rate, disable Add-entry until a rate/override exists, and
translate the backend HOURLY_RATE_REQUIRED toast (en+de).

backend/migrations/core/113_add_default_hourly_rate.js

@@ -0,0 +1,35 @@
+/**
+ * Migration: install-wide default hourly rate.
+ *
+ * Background: hour entries resolve a billing rate through
+ *   entry.hourly_rate_minor_override → customer.hourly_rate_minor.
+ * When a customer had neither set, saving an entry hard-failed with
+ * HOURLY_RATE_REQUIRED — a confusing save-time error on the hours page.
+ * This adds an install-wide fallback so a single global rate covers
+ * every customer who hasn't been given an individual one. The chain
+ * becomes:
+ *   entry override → customer rate → business_profile default → (CTA).
+ *
+ * Stored in minor units (matches customer_accounts.hourly_rate_minor).
+ * Nullable, default NULL: existing installs keep today's behaviour
+ * (no implicit rate) until the admin sets one — no surprise rate gets
+ * applied on upgrade (migration-preserve-existing-state guidance).
+ *
+ * Idempotent: guarded by hasColumn so a re-run is a no-op.
+ */
+
+exports.up = async function(knex) {
+  if (!(await knex.schema.hasTable('business_profile'))) return;
+  if (await knex.schema.hasColumn('business_profile', 'default_hourly_rate_minor')) return;
+  await knex.schema.alterTable('business_profile', (table) => {
+    table.bigInteger('default_hourly_rate_minor');
+  });
+};
+
+exports.down = async function(knex) {
+  if (!(await knex.schema.hasTable('business_profile'))) return;
+  if (!(await knex.schema.hasColumn('business_profile', 'default_hourly_rate_minor'))) return;
+  await knex.schema.alterTable('business_profile', (table) => {
+    table.dropColumn('default_hourly_rate_minor');
+  });
+};