fix(calendar): preserve manual picks when regenerating a month (#2)

generatePlan gains a preAssigned parameter: manual entries occupy slots,
count toward eligibility (maxPerMonth/minGap) and are never double-booked;
only the remaining slots per day are auto-filled. The generate handler now
keeps manual:true entries for the month instead of wiping them.

Adds tests for slot occupation, fully-manual days, and maxPerMonth counting.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

js/calendar.js

@@ -123,12 +123,15 @@ export function renderCalendar() {
     const workdays = getWorkdays(currentYear, currentMonth,
       state.calendar.holidays, state.calendar.companyClosures);
     const prefix = currentYear + '-' + String(currentMonth).padStart(2, '0');
+    // Preserve manual picks for this month; only auto-entries get regenerated.
+    const manualEntries = state.history.filter(h => h.date.startsWith(prefix) && h.manual);
     const historyWithoutMonth = state.history.filter(h => !h.date.startsWith(prefix));
-    // Reset month entries from history so dedup-by-(date,employeeId) doesn't keep stale rows
-    state.history = historyWithoutMonth;
-    const result = generatePlan(workdays, state.employees, historyWithoutMonth, Math.random, state.settings.moderatorsPerDay);
-    currentPlan = result.assignments;
-    addHistoryEntries(state, currentPlan);
+    // Drop stale auto-entries for the month, keep history + manual picks
+    state.history = historyWithoutMonth.concat(manualEntries);
+    const result = generatePlan(workdays, state.employees, historyWithoutMonth,
+      Math.random, state.settings.moderatorsPerDay, manualEntries);
+    addHistoryEntries(state, result.assignments);
+    currentPlan = manualEntries.concat(result.assignments);
     saveState();
     renderSkippedWarning(warnBox, result.skippedDates);
     renderGrid(wrapper, exportBtn);