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/src/lib/householdTimezone.ts

@@ -16,7 +16,7 @@ import { appConfig } from '../db/schema.js';
 
 /**
  * Returns the stored household timezone from app_config, or falls back to:
- *   1. process.env.TZ (if set and non-empty)
+ *   1. process.env.TZ (only if set and non-empty — empty/whitespace is ignored, WR-01)
  *   2. Intl.DateTimeFormat().resolvedOptions().timeZone
  *
  * This is the D-05 single source of truth for the server-side all-day "9 AM local"
@@ -31,11 +31,33 @@ export async function getHouseholdTimezone(
     .where(eq(appConfig.key, 'household_timezone'))
     .limit(1);
 
-  return (
-    row?.value ??
-    process.env.TZ ??
-    Intl.DateTimeFormat().resolvedOptions().timeZone
-  );
+  return resolveHouseholdTimezone(row?.value ?? null);
+}
+
+/**
+ * Resolves the household timezone from an already-fetched stored value, applying
+ * the D-06 fallback chain. Centralizing the policy here (D-05) means callers that
+ * have already read the row — e.g. the GET /config/timezone handler — can reuse it
+ * without a second DB round-trip (IN-01), and there is exactly one place where the
+ * fallback rules live (IN-02).
+ *
+ * WR-01: `??` only short-circuits on null/undefined, so a set-but-empty
+ * `process.env.TZ` (`TZ=` or `TZ='   '`) would otherwise leak through and yield an
+ * invalid IANA zone that throws inside `Intl.DateTimeFormat({ timeZone })` downstream.
+ * Empty/whitespace-only candidate values are treated as absent so they fall through.
+ */
+export function resolveHouseholdTimezone(storedValue: string | null): string {
+  const stored = storedValue?.trim();
+  if (stored) {
+    return stored;
+  }
+
+  const envTz = process.env.TZ?.trim();
+  if (envTz) {
+    return envTz;
+  }
+
+  return Intl.DateTimeFormat().resolvedOptions().timeZone;
 }
 
 /**