Fix monthly trend chart — rolling 12-month average with LY comparison

Replace raw monthly totals with a proper rolling 12-month average trend.
Fetches 36 months of data; the 12 display points each show the average
of the 12 months ending on that point (TY) and the same window shifted
back a year (LY), producing a smooth upward trend vs last year.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/WeeklyActual.tsx

@@ -349,7 +349,7 @@ function MonthProgressSection({
         </div>
 
         <div className="wa-chart-card">
-          <h4>Monthly Average Turnover — Last 12 Months</h4>
+          <h4>Rolling 12-Month Avg Turnover — Trend</h4>
           <ResponsiveContainer width="100%" height={220}>
             <LineChart data={trend} margin={{ top: 8, right: 8, left: 8, bottom: 0 }}>
               <CartesianGrid strokeDasharray="3 3" stroke="#e5e7eb" />
@@ -357,9 +357,8 @@ function MonthProgressSection({
               <YAxis tickFormatter={v => `£${(v/1000).toFixed(0)}k`} tick={{ fontSize: 10 }} width={52} />
               <Tooltip formatter={fmtChartCcy} />
               <Legend iconSize={10} wrapperStyle={{ fontSize: 11 }} />
-              <Line type="monotone" dataKey="rooms" stroke={CHART_COLORS.rooms} strokeWidth={2} dot={false} name="Rooms" />
-              <Line type="monotone" dataKey="dry"   stroke={CHART_COLORS.dry}   strokeWidth={2} dot={false} name="Dry" />
-              <Line type="monotone" dataKey="wet"   stroke={CHART_COLORS.wet}   strokeWidth={2} dot={false} name="Wet" />
+              <Line type="monotone" dataKey="this_year_avg" stroke={CHART_COLORS.thisWeek} strokeWidth={2.5} dot={false} name="This Year" />
+              <Line type="monotone" dataKey="last_year_avg" stroke={CHART_COLORS.budget}   strokeWidth={2}   dot={false} name="Last Year" strokeDasharray="5 3" />
             </LineChart>
           </ResponsiveContainer>
         </div>

frontend/src/types.ts

@@ -101,11 +101,9 @@ export interface MonthProgress {
 }
 
 export interface MonthTrend {
-  label: string
-  rooms: number
-  dry:   number
-  wet:   number
-  total: number
+  label:         string
+  this_year_avg: number
+  last_year_avg: number
 }
 
 export interface WeeklyActualData {