chore(dashboard): fix stale trend-chart docstring + dedupe All-range test mock

Adversarial-review follow-ups to PR #1257:

- frontend/src/dashboard.ts: the loadSavingsTrendChart docstring still
  described the pre-QA-2.3 "empty windows render labelled axes" behaviour;
  the shipped code (lines 1083-1098) shows a filter-aware empty-state
  banner instead. Reconciled the docstring to describe the actual three
  branches (data / empty / fetch-error).

- frontend/src/__tests__/dashboard.test.ts: the new "All range sends ~365d
  start" test set mockResolvedValue twice with a mockClear() in between.
  mockClear() resets call history but NOT mock return values, so the first
  set was dead code. Collapsed to a single mockResolvedValue + mockClear
  with a clarifying comment.

97 dashboard tests pass, tsc clean. No behaviour change.

Author: cristim

frontend/src/__tests__/dashboard.test.ts

@@ -884,13 +884,12 @@ describe('Dashboard Module', () => {
       bAll.textContent = 'All';
       document.body.appendChild(bAll);
       setupSavingsTrendHandlers();
+      // Override the beforeEach default of empty data_points so the chart
+      // actually renders -- the test asserts the empty-state stays hidden.
       (api.getSavingsAnalytics as jest.Mock).mockResolvedValue({
         data_points: [{ timestamp: new Date().toISOString(), cumulative_savings: 500, total_savings: 50, total_upfront: 0, purchase_count: 1 }],
       });
       (api.getSavingsAnalytics as jest.Mock).mockClear();
-      (api.getSavingsAnalytics as jest.Mock).mockResolvedValue({
-        data_points: [{ timestamp: new Date().toISOString(), cumulative_savings: 500, total_savings: 50, total_upfront: 0, purchase_count: 1 }],
-      });
 
       bAll.click();
       await new Promise(r => setTimeout(r, 0));

frontend/src/dashboard.ts

@@ -1003,11 +1003,14 @@ export function formatTrendAxisTick(tsMs: number, intervalHint: 'hourly' | 'dail
 
 /**
  * Load the savings-over-time trend chart for the dashboard. Fetches the
- * history analytics endpoint with the currently-selected range and
- * renders a line chart of cumulative savings spanning the full selected
- * window on the x-axis (QA row 405, step 3.1). Empty windows render
- * labelled axes rather than a "no data" stub; only fetch failures use
- * the error stub.
+ * history analytics endpoint with the currently-selected range and renders
+ * a line chart of cumulative savings spanning the full selected window on
+ * the x-axis (QA row 405, step 3.1). When the response is empty (QA 2.3
+ * supersedes QA 3.1's earlier empty-axes suggestion), the canvas is hidden
+ * and an empty-state banner is shown: a filter-aware "No savings history
+ * for <filter>" when a provider or account chip is active, otherwise the
+ * generic "No purchase history yet." Fetch failures hide the canvas and
+ * show the "Savings history is not available yet." stub instead.
  */
 export async function loadSavingsTrendChart(): Promise<void> {
   const canvas = document.getElementById('savings-trend-chart') as HTMLCanvasElement | null;