refactor(admin): use CSS custom properties for bar chart heights

Refactors the admin dashboard bar charts to use CSS custom properties (`--bar-height`) instead of fragile inline `height` styles.

Changes include:
- Updating the HTML templates to pass the height as a CSS variable.
- Converting the `.bar-chart` layout from Flexbox to CSS Grid for more consistent column distribution.
- Using absolute positioning for `.bar-chart-bar` inside `.bar-chart-track`.
- Adding a Go test to verify that the dashboard renders the CSS variable and no longer uses inline height styles.

backend/libs/handlers/accounts_test.go

@@ -724,6 +724,28 @@ func TestAdminOverviewChartsUseZeroAndFullHeights(t *testing.T) {
 	}
 }
 
+func TestAdminOverviewRendersBarHeightVariables(t *testing.T) {
+	app, cleanup := newTestApp(t)
+	defer cleanup()
+	adminToken := createAdminSession(t, app)
+	uploadThroughApp(t, app)
+
+	request := httptest.NewRequest(http.MethodGet, "/admin", nil)
+	request.AddCookie(&http.Cookie{Name: userSessionCookieName, Value: adminToken})
+	response := httptest.NewRecorder()
+	app.AdminDashboard(response, request)
+	if response.Code != http.StatusOK {
+		t.Fatalf("AdminDashboard status = %d, body = %s", response.Code, response.Body.String())
+	}
+	body := response.Body.String()
+	if !strings.Contains(body, "--bar-height: 100%") {
+		t.Fatalf("admin overview did not render a full-height bar: %s", body)
+	}
+	if strings.Contains(body, `style="height:`) {
+		t.Fatalf("admin overview still uses fragile percent height styles: %s", body)
+	}
+}
+
 func TestAdminStorageProviderPagesOnlyRenderRelevantFields(t *testing.T) {
 	app, cleanup := newTestApp(t)
 	defer cleanup()