refactor(admin): use inline pixel heights for overview charts

Refactors the admin overview dashboard charts to use inline pixel heights (up to 150px) instead of CSS variables and percentage-based heights. This provides more robust rendering and layout control.

Changes include:
- Replacing `Height` with `HeightPx` in chart bar structures.
- Rendering inline styles for height and width on charts and status bars.
- Adding fallback data attributes (`data-height-px`, `data-chart-value`, etc.) and loading a new fallback script (`25-admin-charts.js`).
- Updating and expanding test coverage to assert correct scaling and HTML rendering.

backend/libs/handlers/admin.go

@@ -159,15 +159,17 @@ type adminOverview struct {
 }
 
 type adminChartBar struct {
-	Label  string
-	Value  string
-	Height int // 0-100, percent of the tallest bar
+	Label    string
+	Value    string
+	HeightPx int
+	RawValue int64
 }
 
 type adminStatBar struct {
-	Label   string
-	Value   string
-	Percent int
+	Label        string
+	Value        string
+	RawValue     int
+	WidthPercent int
 }
 
 type adminBoxView struct {
@@ -336,6 +338,7 @@ func (a *App) recentBoxViews(boxes []services.AdminBox, limit int) []adminBoxVie
 // status distributions for the overview dashboard.
 func buildAdminOverview(boxes []services.AdminBox, stats services.AdminStats) adminOverview {
 	const days = 14
+	const chartMaxHeightPx = 150
 	now := time.Now().UTC()
 	today := time.Date(now.Year(), now.Month(), now.Day(), 0, 0, 0, 0, time.UTC)
 
@@ -374,14 +377,16 @@ func buildAdminOverview(boxes []services.AdminBox, stats services.AdminStats) ad
 	storageDays := make([]adminChartBar, days)
 	for i := 0; i < days; i++ {
 		uploadDays[i] = adminChartBar{
-			Label:  labels[i],
-			Value:  strconv.Itoa(counts[i]),
-			Height: scaleHeight(int64(counts[i]), int64(maxCount)),
+			Label:    labels[i],
+			Value:    strconv.Itoa(counts[i]),
+			HeightPx: scaleHeightPx(int64(counts[i]), int64(maxCount), chartMaxHeightPx),
+			RawValue: int64(counts[i]),
 		}
 		storageDays[i] = adminChartBar{
-			Label:  labels[i],
-			Value:  helpers.FormatBytes(bytes[i]),
-			Height: scaleHeight(bytes[i], maxBytes),
+			Label:    labels[i],
+			Value:    helpers.FormatBytes(bytes[i]),
+			HeightPx: scaleHeightPx(bytes[i], maxBytes, chartMaxHeightPx),
+			RawValue: bytes[i],
 		}
 	}
 
@@ -389,10 +394,11 @@ func buildAdminOverview(boxes []services.AdminBox, stats services.AdminStats) ad
 	if activeBoxes < 0 {
 		activeBoxes = 0
 	}
+	maxStatusValue := maxInt(activeBoxes, stats.ExpiredBoxes, stats.ProtectedBoxes)
 	statusBars := []adminStatBar{
-		{Label: "Active", Value: strconv.Itoa(activeBoxes), Percent: percentOf(activeBoxes, stats.TotalBoxes)},
-		{Label: "Expired", Value: strconv.Itoa(stats.ExpiredBoxes), Percent: percentOf(stats.ExpiredBoxes, stats.TotalBoxes)},
-		{Label: "Password-protected", Value: strconv.Itoa(stats.ProtectedBoxes), Percent: percentOf(stats.ProtectedBoxes, stats.TotalBoxes)},
+		{Label: "Active", Value: strconv.Itoa(activeBoxes), RawValue: activeBoxes, WidthPercent: percentOf(activeBoxes, maxStatusValue)},
+		{Label: "Expired", Value: strconv.Itoa(stats.ExpiredBoxes), RawValue: stats.ExpiredBoxes, WidthPercent: percentOf(stats.ExpiredBoxes, maxStatusValue)},
+		{Label: "Password-protected", Value: strconv.Itoa(stats.ProtectedBoxes), RawValue: stats.ProtectedBoxes, WidthPercent: percentOf(stats.ProtectedBoxes, maxStatusValue)},
 	}
 
 	return adminOverview{
@@ -402,13 +408,16 @@ func buildAdminOverview(boxes []services.AdminBox, stats services.AdminStats) ad
 	}
 }
 
-func scaleHeight(value, max int64) int {
+func scaleHeightPx(value, max int64, maxHeightPx int) int {
 	if max <= 0 || value <= 0 {
 		return 0
 	}
-	height := int(value * 100 / max)
-	if height < 4 {
-		height = 4
+	height := int(value * int64(maxHeightPx) / max)
+	if height < 8 {
+		height = 8
+	}
+	if height > maxHeightPx {
+		return maxHeightPx
 	}
 	return height
 }
@@ -420,6 +429,16 @@ func percentOf(value, total int) int {
 	return value * 100 / total
 }
 
+func maxInt(values ...int) int {
+	max := 0
+	for _, value := range values {
+		if value > max {
+			max = value
+		}
+	}
+	return max
+}
+
 func (a *App) AdminUsers(w http.ResponseWriter, r *http.Request) {
 	if !a.requireAdmin(w, r) {
 		return