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/image/extract.go

@@ -15,10 +15,24 @@ type ExtractResult struct {
 	InitrdFilename string
 }
 
+type ProgressFunc func(stage string, detail string)
+
 var kernelCandidates = []string{"linux26", "vmlinuz", "bzImage"}
 var initrdCandidates = []string{"initrd.img", "initrd", "initrd.gz"}
 
 func ExtractFromISO(r io.Reader, destDir string) (*ExtractResult, error) {
+	return ExtractFromISOWithProgress(r, destDir, nil)
+}
+
+func ExtractFromISOWithProgress(r io.Reader, destDir string, progress ProgressFunc) (*ExtractResult, error) {
+	report := func(stage, detail string) {
+		if progress != nil {
+			progress(stage, detail)
+		}
+	}
+
+	report("receiving", "Writing ISO to disk...")
+
 	tmp, err := os.CreateTemp(filepath.Dir(destDir), "iso-upload-*.tmp")
 	if err != nil {
 		return nil, fmt.Errorf("create temp file: %w", err)
@@ -32,6 +46,8 @@ func ExtractFromISO(r io.Reader, destDir string) (*ExtractResult, error) {
 	}
 	tmp.Close()
 
+	report("parsing", "Parsing ISO image...")
+
 	f, err := os.Open(tmpPath)
 	if err != nil {
 		return nil, fmt.Errorf("open temp ISO: %w", err)
@@ -69,13 +85,20 @@ func ExtractFromISO(r io.Reader, destDir string) (*ExtractResult, error) {
 		return nil, fmt.Errorf("no initrd found in ISO (looked for %s)", strings.Join(initrdCandidates, ", "))
 	}
 
+	report("extracting", "Extracting kernel...")
+
 	if err := extractFile(kernelFile, filepath.Join(destDir, kernelName)); err != nil {
 		return nil, fmt.Errorf("extract kernel: %w", err)
 	}
+
+	report("extracting", "Extracting initrd...")
+
 	if err := extractFile(initrdFile, filepath.Join(destDir, initrdName)); err != nil {
 		return nil, fmt.Errorf("extract initrd: %w", err)
 	}
 
+	report("complete", "Extraction complete")
+
 	return &ExtractResult{
 		KernelFilename: kernelName,
 		InitrdFilename: initrdName,