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/__tests__/services/customerHoursService.test.js

@@ -58,13 +58,30 @@ describe('resolveEffectiveRate', () => {
     )).toBe(15000);
   });
 
-  it('throws when both override and customer rate are unset', () => {
+  it('throws when override, customer rate, AND install default are all unset', () => {
     expect(() => resolveEffectiveRate(
       { hourly_rate_minor_override: null },
       { hourly_rate_minor: null },
+      null,
     )).toThrow(/No hourly rate/);
   });
 
+  it('falls back to the install-wide default when override + customer rate are unset', () => {
+    expect(resolveEffectiveRate(
+      { hourly_rate_minor_override: null },
+      { hourly_rate_minor: null },
+      12000,
+    )).toBe(12000);
+  });
+
+  it('customer rate wins over the install-wide default', () => {
+    expect(resolveEffectiveRate(
+      { hourly_rate_minor_override: null },
+      { hourly_rate_minor: 15000 },
+      12000,
+    )).toBe(15000);
+  });
+
   it('treats override=0 as "explicitly zero" (not null)', () => {
     // Override === 0 is unusual but legal — pro bono blocks, internal
     // tracking. Must NOT fall through to the customer default.