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/templates/pages/admin.html

@@ -67,7 +67,7 @@
           {{range .Data.Overview.UploadDays}}
           <div class="bar-chart-col" title="{{.Label}}: {{.Value}}">
             <span class="bar-chart-value">{{.Value}}</span>
-            <span class="bar-chart-track"><span class="bar-chart-bar" style="height: {{.Height}}%"></span></span>
+            <span class="bar-chart-track"><span class="bar-chart-bar" style="--bar-height: {{.Height}}%"></span></span>
             <span class="bar-chart-label">{{.Label}}</span>
           </div>
           {{end}}
@@ -99,7 +99,7 @@
         {{range .Data.Overview.StorageDays}}
         <div class="bar-chart-col" title="{{.Label}}: {{.Value}}">
           <span class="bar-chart-value">{{.Value}}</span>
-          <span class="bar-chart-track"><span class="bar-chart-bar" style="height: {{.Height}}%"></span></span>
+          <span class="bar-chart-track"><span class="bar-chart-bar" style="--bar-height: {{.Height}}%"></span></span>
           <span class="bar-chart-label">{{.Label}}</span>
         </div>
         {{end}}