feat(upload): add resumable chunk configuration and file validation

- Add `WARPBOX_RESUMABLE_CHUNK_MODE` and `WARPBOX_RESUMABLE_CHUNK_PATH` environment variables to configure temporary chunk storage.
- Implement strict file validation for resuming uploads to ensure selected files match the pending session's metadata.
- Add `PLANS.md` to document development stages, roadmap, and API specifications (including batching and resumable flows).

backend/templates/pages/download.html

@@ -24,6 +24,12 @@
       {{end}}
 
       {{if .Data.Files}}
+        {{$processing := false}}{{range .Data.Files}}{{if .Processing}}{{$processing = true}}{{end}}{{end}}
+        {{if $processing}}
+          <div class="upload-processing-alert" role="status">
+            Some files are still processing. You can share this link now, but processing files will become available shortly.
+          </div>
+        {{end}}
         {{$single := eq (len .Data.Files) 1}}
         <div class="badge-row">
           <span class="badge badge-expiry">Expires {{.Data.ExpiresLabel}}</span>
@@ -74,8 +80,8 @@
           </div>
           <div class="download-list file-browser is-thumbs" data-file-browser>
             {{range .Data.Files}}
-              <article class="download-item file-card" data-kind="{{.PreviewKind}}" data-file-context data-preview-url="{{.URL}}" data-view-url="{{.DownloadURL}}?inline=1" data-download-url="{{.DownloadURL}}" data-file-name="{{.Name}}" data-reaction-card data-react-url="{{.ReactURL}}" data-reacted="{{if .Reacted}}true{{else}}false{{end}}">
-                <a class="file-open" href="{{.DownloadURL}}?inline=1"{{if not $single}} target="_blank" rel="noopener noreferrer"{{end}} aria-label="Open {{.Name}}">
+              <article class="download-item file-card {{if .Processing}}is-processing{{end}}" data-kind="{{.PreviewKind}}" {{if not .Processing}}data-file-context data-preview-url="{{.URL}}" data-view-url="{{.DownloadURL}}?inline=1" data-download-url="{{.DownloadURL}}"{{end}} data-file-name="{{.Name}}" data-reaction-card data-react-url="{{.ReactURL}}" data-reacted="{{if .Reacted}}true{{else}}false{{end}}">
+                {{if .Processing}}<div class="file-open" aria-label="{{.Name}} is processing">{{else}}<a class="file-open" href="{{.DownloadURL}}?inline=1"{{if not $single}} target="_blank" rel="noopener noreferrer"{{end}} aria-label="Open {{.Name}}">{{end}}
                   <span class="file-media">
                     {{if .HasThumbnail}}
                       <img class="file-thumb" src="{{.ThumbnailURL}}" alt="" loading="lazy">
@@ -86,11 +92,11 @@
                   </span>
                   <span class="file-main">
                     <strong class="file-name" title="{{.Name}}">{{.Name}}</strong>
-                    <small>{{.Size}} · {{.ContentType}}</small>
+                    <small>{{.Size}} · {{if .Processing}}Processing{{else}}{{.ContentType}}{{end}}</small>
                   </span>
-                  <span class="file-type">{{.ContentType}}</span>
+                  <span class="file-type">{{if .Processing}}Processing{{else}}{{.ContentType}}{{end}}</span>
                   <span class="file-size">{{.Size}}</span>
-                </a>
+                {{if .Processing}}</div>{{else}}</a>{{end}}
                 {{if not $.Data.Locked}}
                   <div class="file-reaction-dock" data-reaction-dock>
                     <div class="file-reactions" data-reaction-list>