Add upload progress bar with SSE extraction status

ISO uploads now show a progress bar during file transfer (via XHR
upload.onprogress) and real-time extraction status (via SSE events
through the existing Hub). Falls back to plain form POST if JS is
disabled.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/api/render.go

@@ -160,7 +160,8 @@ func imageUploadForm(errMsg string) string {
 	return layout("Upload Image", fmt.Sprintf(`
 		<h2>Upload Boot Image</h2>
 		%s
-		<form method="POST" action="/images/upload" enctype="multipart/form-data" class="form">
+		<form id="upload-form" method="POST" action="/images/upload" enctype="multipart/form-data" class="form">
+			<input type="hidden" name="upload_id" id="upload-id" value="">
 			<label>Name<input type="text" name="name" placeholder="proxmox-8.2" required pattern="[a-z0-9][a-z0-9.\-]*"></label>
 			<label>Version<input type="text" name="version" placeholder="8.2-1" required></label>
 			<label>Kind
@@ -169,9 +170,16 @@ func imageUploadForm(errMsg string) string {
 				</select>
 			</label>
 			<label>ISO File<input type="file" name="iso" accept=".iso" required></label>
-			<p style="font-size:0.8rem;color:var(--text-muted);margin-bottom:0.5rem">Upload may take several minutes for large ISOs. The kernel and initrd will be extracted automatically.</p>
 			<button type="submit" class="btn">Upload &amp; Extract</button>
 		</form>
+		<div id="upload-progress" style="display:none" class="upload-progress">
+			<h3 id="progress-title">Uploading ISO...</h3>
+			<div class="progress-bar-track">
+				<div class="progress-bar-fill" id="progress-fill"></div>
+			</div>
+			<div class="progress-text" id="progress-text">Preparing upload...</div>
+			<div class="progress-detail" id="progress-detail"></div>
+		</div>
 	`, errHTML))
 }