fix(backend): correct "next weekday" and harden resolvers against model output

Four defects found by review of the preceding commits.

"next <weekday>" was occurrence-anchored ("this" plus seven) rather than week-
anchored. Said on a Thursday, "Thursday next week" resolved to +14 instead of
+7: next week runs Sat 10-18 to Fri 10-24, so its Thursday is 10-23, not 10-30.
A lab case a week late. "next" now counts from the start of the following
Saturday-start week, which also lets "this" and "next" correctly coincide —
said on a Thursday, "the coming Saturday" and "Saturday next week" are the same
day. "this" stays occurrence-anchored so it can never resolve into the past.

The other three all come from the same root cause: exported functions that are
reachable from untrusted model output must degrade, not throw or drop.

- a non-object `due` (the model emitting a bare string) was treated as "no
  deadline spoken" and silently discarded; only null/undefined mean absent now,
  anything else is flagged so the clinician sees something was heard and lost
- isJalaliLeapYear / jalaliDaysInMonth threw for years outside the conversion
  table, contradicting the module's own "degrade to null" contract; they now
  return false / 0, which also makes isValidJalaliDate's day check naturally
  false
- civilDateInZone passed a client-supplied zone straight to Intl, which raises
  RangeError before any fallback; it now validates and backstops to UTC, so a
  bad zone costs at most a day rather than a 500

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/src/modules/voice/due-date.resolver.ts

@@ -92,11 +92,25 @@ function describe(intent: DueIntent): string {
   }
 }
 
+/** The Iranian week starts Saturday. */
+const WEEK_START_JS = WEEKDAY_TO_JS.saturday;
+
+/** Most recent Saturday, counting today if today is Saturday. */
+function startOfWeek(iso: string): string {
+  const back = (civilDateJsWeekday(iso) - WEEK_START_JS + 7) % 7;
+  return addDays(iso, -back);
+}
+
 /**
- * `which: 'this'` means the soonest occurrence strictly after today, so "by Thursday" said
- * on a Thursday means the next one — a deadline of today is almost never what was meant.
- * `'next'` adds a further week. The Iranian week starts Saturday, which this arithmetic is
- * agnostic to (it counts forward from today), but the tests pin it explicitly.
+ * `'this'` is occurrence-anchored: the soonest occurrence strictly after today, so "by
+ * Thursday" said on a Thursday means the next one — a deadline of today is almost never
+ * what was meant, and this can never resolve into the past.
+ *
+ * `'next'` is *week*-anchored, not "this plus seven". "Thursday next week" means the
+ * Thursday of the Saturday-start week after this one; adding a week to `'this'` would
+ * overshoot by seven days whenever `'this'` had already rolled into next week. The two
+ * can legitimately coincide — said on a Thursday, "the coming Saturday" and "Saturday
+ * next week" are the same day.
  */
 function resolveWeekday(
   intent: Extract<DueIntent, { kind: 'weekday' }>,
@@ -104,13 +118,20 @@ function resolveWeekday(
 ) {
   const targetJs = WEEKDAY_TO_JS[intent.weekday];
   if (targetJs === undefined) return null;
-  if (intent.which !== 'this' && intent.which !== 'next') return null;
 
-  const todayJs = civilDateJsWeekday(todayIso);
-  let delta = (targetJs - todayJs + 7) % 7;
-  if (delta === 0) delta = 7;
-  if (intent.which === 'next') delta += 7;
-  return addDays(todayIso, delta);
+  if (intent.which === 'this') {
+    const todayJs = civilDateJsWeekday(todayIso);
+    let delta = (targetJs - todayJs + 7) % 7;
+    if (delta === 0) delta = 7;
+    return addDays(todayIso, delta);
+  }
+
+  if (intent.which === 'next') {
+    const offsetInWeek = (targetJs - WEEK_START_JS + 7) % 7;
+    return addDays(startOfWeek(todayIso), 7 + offsetInWeek);
+  }
+
+  return null;
 }
 
 function resolveOffset(
@@ -132,9 +153,15 @@ export function resolveDueDate(
   intent: DueIntent | null | undefined,
   todayIso: string,
 ): DueResolution {
-  if (!intent || typeof intent !== 'object') {
+  // Absent is not an error — most utterances carry no deadline. Anything else that is not
+  // an intent object is a deadline we failed to understand, and must be flagged rather
+  // than silently dropped.
+  if (intent === null || intent === undefined) {
     return { dueDate: null, unresolved: null };
   }
+  if (typeof intent !== 'object') {
+    return unresolved(String(intent).slice(0, 120));
+  }
   if (!isRealCivilDate(todayIso)) {
     return unresolved(describe(intent));
   }