feat(config): allow -1 to represent unlimited upload limits

Introduce support for configuring unlimited upload limits by allowing -1
as a valid value for anonymous and user upload MB limits.

Changes include:
- Added `envMegabytesLimitFloat` and helper functions to parse and validate limits where -1 is allowed.
- Updated validation logic to accept -1 for `AnonymousMaxUploadMB`, `AnonymousDailyUploadMB`, and `UserDailyUploadMB`.
- Added a test case to verify unlimited upload policy behavior.

backend/templates/pages/admin_settings.html

@@ -34,7 +34,7 @@
         <div class="table-header">
           <div>
             <h2>Upload policy</h2>
-            <p>Admin users bypass all upload caps. Values are in megabytes.</p>
+            <p>Admin users bypass all upload caps. Values are in megabytes; use -1 for unlimited upload size or daily upload caps.</p>
           </div>
         </div>
 

backend/templates/pages/admin_user_edit.html

@@ -38,7 +38,7 @@
       </div>
     {{end}}
 
-    <div class="metric-grid">
+    <div class="metric-grid user-edit-metrics">
       <article class="metric-card"><span>Storage used</span><strong>{{.Data.UserEdit.StorageUsed}}</strong></article>
       <article class="metric-card"><span>Uploaded today</span><strong>{{.Data.UserEdit.DailyUsed}}</strong></article>
       <article class="metric-card"><span>Effective quota</span><strong>{{.Data.UserEdit.EffectiveStorage}}</strong></article>
@@ -50,7 +50,7 @@
         <div class="table-header">
           <div>
             <h2>Identity and limits</h2>
-            <p>Blank limit fields inherit the global user defaults. Storage quota set to 0 means unlimited.</p>
+            <p>Blank limit fields inherit the global user defaults. Use -1 for unlimited upload size or daily upload caps. Storage quota set to 0 means unlimited.</p>
           </div>
         </div>
         <form class="settings-form" action="/admin/users/{{.Data.UserEdit.ID}}/edit" method="post">