fix(frontend): route dashboard chart tooltips through formatCurrency

dashboard.ts's Chart.js tooltip and axis-tick callbacks built dollar
strings by hand with `$${x.toLocaleString()}`, six occurrences across
two charts. Same unpinned-locale defect as formatCurrency() before
the previous commit -- these fell back to the host locale too -- but
the deeper issue was duplication: dashboard.ts already imports
formatCurrency from utils.ts and uses it elsewhere in the same file,
so these six were reinventing it inline instead of calling it.

Routes all six through the shared helper. Mechanical: each site
formats a guaranteed-finite number (never null/undefined) with no
fraction digits and a literal '$' prefix, exactly formatCurrency's
default signature, so the change is a straight substitution with no
behavior difference beyond the locale fix itself.

These call sites render into a Chart.js <canvas>, not the DOM, so no
existing test exercises them and none are being added here -- the
change is verified by tsc, eslint, and the full suite passing under
both locales, not by new coverage of previously-untested chart code.

Author: cristim

frontend/src/dashboard.ts

@@ -962,7 +962,7 @@ export function renderSavingsByService(
               // tooltip; the range datasets share the full breakdown.
               if (ctx.dataset?.label === 'Current / Committed') {
                 const current = byService[svc]?.current_savings ?? 0;
-                return `Current / Committed: $${current.toLocaleString()}`;
+                return `Current / Committed: ${formatCurrency(current)}`;
               }
               const current = byService[svc]?.current_savings ?? 0;
               const maxRec = s?.max ?? 0;
@@ -973,10 +973,10 @@ export function renderSavingsByService(
               const pct = totalSavings > 0 ? ((total / totalSavings) * 100).toFixed(1) : '0.0';
               const lines = [
                 `Service: ${svc}`,
-                `Total: $${total.toLocaleString()} (${pct}% of all services)`,
-                `Current / Committed: $${current.toLocaleString()}`,
-                `Lowest option: $${lowestOption.toLocaleString()}`,
-                `Upside: $${upside.toLocaleString()}`,
+                `Total: ${formatCurrency(total)} (${pct}% of all services)`,
+                `Current / Committed: ${formatCurrency(current)}`,
+                `Lowest option: ${formatCurrency(lowestOption)}`,
+                `Upside: ${formatCurrency(upside)}`,
               ];
               if (s?.minLabel) lines.push(`Min option: ${s.minLabel}`);
               if (s?.maxLabel) lines.push(`Max option: ${s.maxLabel}`);
@@ -994,7 +994,7 @@ export function renderSavingsByService(
           stacked: true,
           beginAtZero: true,
           title: { display: true, text: 'Monthly savings ($)' },
-          ticks: { callback: (v) => '$' + (v as number).toLocaleString() },
+          ticks: { callback: (v) => formatCurrency(v as number) },
         },
       },
     },
@@ -1131,7 +1131,7 @@ export async function loadSavingsTrendChart(): Promise<void> {
                 const raw = items[0]?.raw as { x: number; y: number } | undefined;
                 return raw?.x != null ? formatTrendAxisTick(raw.x, interval) : '';
               },
-              label: (ctx) => `Cumulative savings: $${((ctx.raw as { x: number; y: number }).y).toLocaleString()}`,
+              label: (ctx) => `Cumulative savings: ${formatCurrency((ctx.raw as { x: number; y: number }).y)}`,
             },
           },
         },
@@ -1147,7 +1147,7 @@ export async function loadSavingsTrendChart(): Promise<void> {
           },
           y: {
             beginAtZero: true,
-            ticks: { callback: (v) => '$' + (v as number).toLocaleString() },
+            ticks: { callback: (v) => formatCurrency(v as number) },
           },
         },
       },