fix(18): WR-01 treat empty/blank TZ as unset in household timezone fallback

The D-06 fallback used row?.value ?? process.env.TZ ?? Intl..., but ??
only short-circuits on null/undefined. A set-but-empty TZ ('' or '   ')
leaked through and yielded an invalid IANA zone that throws inside
Intl.DateTimeFormat({ timeZone }) downstream, silently dropping the
all-day reminder. Extract resolveHouseholdTimezone() which trims and
treats empty/whitespace candidate values (stored value and TZ) as
absent so they fall through to the Intl resolved zone. Adds RED->GREEN
unit tests for empty and whitespace-only TZ.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/api/tests/lib/householdTimezone.test.ts

@@ -97,6 +97,24 @@ describe('getHouseholdTimezone', () => {
     const result = await getHouseholdTimezone(mockDb as never);
     expect(result).toBe('Europe/London');
   });
+
+  it('treats an empty process.env.TZ as unset and falls through to the Intl zone (WR-01)', async () => {
+    mockDb.select.mockReturnValue(makeSelectChain([]));
+    process.env.TZ = '';
+
+    const expected = Intl.DateTimeFormat().resolvedOptions().timeZone;
+    const result = await getHouseholdTimezone(mockDb as never);
+    expect(result).toBe(expected);
+  });
+
+  it('treats a whitespace-only process.env.TZ as unset and falls through to the Intl zone (WR-01)', async () => {
+    mockDb.select.mockReturnValue(makeSelectChain([]));
+    process.env.TZ = '   ';
+
+    const expected = Intl.DateTimeFormat().resolvedOptions().timeZone;
+    const result = await getHouseholdTimezone(mockDb as never);
+    expect(result).toBe(expected);
+  });
 });
 
 describe('isValidIanaTimezone', () => {