(refactor + bugfix) Improve data structure and handle empty database edge cases

Refactored months data structure from object to array for better performance
and cleaner iteration. Fixed crash when availableYears array is empty by
adding proper guards and fallback to current year.

Changes:
- MonthLocationList: Changed months prop from object to array type
- HomePage: Refactored reduce logic to build array instead of object
- HomePage: Added empty database handling in year selection logic
- HomePage: Added early returns for invalid year params in empty DB

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

web-app/app/ui/MonthLocationList.tsx

@@ -22,22 +22,22 @@ const getNextYearMonth = (yearMonth:YearMonth) => {
     } as YearMonth);
 }
 
+export type MonthArray =  Array<{
+    yearMonth: YearMonth;
+    locations: BillingLocation[];
+    payedTotal: number;
+    unpaidTotal: number;
+}>;
+
 export interface MonthLocationListProps {
-    availableYears?: number[];
-    months?: {
-        [key: string]: {
-            yearMonth: YearMonth;
-            locations: BillingLocation[];
-            payedTotal: number;
-            unpaidTotal: number;
-        };
-    };
+    availableYears: number[];
+    months: MonthArray;
     userSettings?: UserSettings | null;
 }
 
 export const MonthLocationList:React.FC<MonthLocationListProps  > = ({
     availableYears,
-    months,
+    months: activeYearMonths,
     userSettings,
 }) => {
 
@@ -93,7 +93,7 @@ export const MonthLocationList:React.FC<MonthLocationListProps  > = ({
         }
     }, [search, router, t]);
 
-    if(!availableYears || !months) {
+    if(availableYears.length === 0 || activeYearMonths.length === 0) {
         const currentYearMonth:YearMonth = {
             year: new Date().getFullYear(),
             month: new Date().getMonth() + 1
@@ -107,8 +107,6 @@ export const MonthLocationList:React.FC<MonthLocationListProps  > = ({
         </>)
     };
 
-    const monthsArray = Object.entries(months);
-
     // when the month is toggled, update the URL
     // and set the the new expandedMonth
     const handleMonthToggle = (yearMonth:YearMonth) => {
@@ -123,10 +121,10 @@ export const MonthLocationList:React.FC<MonthLocationListProps  > = ({
     }
 
     return(<>
-            <AddMonthButton yearMonth={getNextYearMonth(monthsArray[0][1].locations[0].yearMonth)} />
+            <AddMonthButton yearMonth={getNextYearMonth(activeYearMonths[0].locations[0].yearMonth)} />
             {
-                monthsArray.map(([monthKey, { yearMonth, locations, unpaidTotal, payedTotal }], monthIx) =>
-                    <MonthCard yearMonth={yearMonth} key={`month-${monthKey}`} unpaidTotal={unpaidTotal} payedTotal={payedTotal} currency={userSettings?.currency} expanded={ yearMonth.month === expandedMonth } onToggle={handleMonthToggle} >
+                activeYearMonths.map(({ yearMonth, locations, unpaidTotal, payedTotal }, monthIx) =>
+                    <MonthCard yearMonth={yearMonth} key={`month-${yearMonth.year}-${yearMonth.month}`} unpaidTotal={unpaidTotal} payedTotal={payedTotal} currency={userSettings?.currency} expanded={ yearMonth.month === expandedMonth } onToggle={handleMonthToggle} >
                         {
                             yearMonth.month === expandedMonth ?
                                 locations.map((location, ix) => <LocationCard key={`location-${location._id}`} location={location} currency={userSettings?.currency} />)