feat(webui): show GPU serial in NVIDIA selection pickers

Every GPU-selection picker (Load/SAT, Burn, Benchmark) now renders the
card serial: "GPU N — <model> · <mem> MiB · sn: <serial>". The serial is
monospace and the digits that differ across the listed GPUs (common
prefix/suffix stripped) are emphasised so operators can tell cards apart.

Row markup was duplicated across three pages (and twice within
page_validate.go). Consolidated into a single module,
internal/webui/gpu_picker.go: beeGpuPicker.render({...}) builds every row;
gpuPickerCSS/gpuPickerJS are injected once by layoutHead/renderPage. Pages
keep their own selection-note text and multi-GPU toggles but no longer
hand-build <label> markup.

ListNvidiaGPUs() adds the serial via nvidia-smi --query-gpu=...,serial;
N/A is normalised to empty. Serial flows to the client unchanged through
the existing /api/gpu/nvidia JSON response.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AudE3Q2nd9kxxVuxPKcTho

audit/internal/platform/sat_gpu_packs.go

@@ -14,6 +14,7 @@ type NvidiaGPU struct {
 	Index    int    `json:"index"`
 	Name     string `json:"name"`
 	MemoryMB int    `json:"memory_mb"`
+	Serial   string `json:"serial,omitempty"`
 }
 
 type NvidiaGPUStatus struct {
@@ -226,7 +227,7 @@ func amdStressJobs(seconds int, cfgFile string) []satJob {
 // ListNvidiaGPUs returns GPUs visible to nvidia-smi.
 func (s *System) ListNvidiaGPUs() ([]NvidiaGPU, error) {
 	out, err := exec.Command("nvidia-smi",
-		"--query-gpu=index,name,memory.total",
+		"--query-gpu=index,name,memory.total,serial",
 		"--format=csv,noheader,nounits").Output()
 	if err != nil {
 		return nil, fmt.Errorf("nvidia-smi: %w", err)
@@ -237,8 +238,8 @@ func (s *System) ListNvidiaGPUs() ([]NvidiaGPU, error) {
 		if line == "" {
 			continue
 		}
-		parts := strings.SplitN(line, ", ", 3)
-		if len(parts) != 3 {
+		parts := strings.SplitN(line, ", ", 4)
+		if len(parts) < 3 {
 			continue
 		}
 		idx, err := strconv.Atoi(strings.TrimSpace(parts[0]))
@@ -246,10 +247,18 @@ func (s *System) ListNvidiaGPUs() ([]NvidiaGPU, error) {
 			continue
 		}
 		memMB, _ := strconv.Atoi(strings.TrimSpace(parts[2]))
+		serial := ""
+		if len(parts) == 4 {
+			serial = strings.TrimSpace(parts[3])
+			if strings.EqualFold(serial, "N/A") || strings.EqualFold(serial, "[N/A]") {
+				serial = ""
+			}
+		}
 		gpus = append(gpus, NvidiaGPU{
 			Index:    idx,
 			Name:     strings.TrimSpace(parts[1]),
 			MemoryMB: memMB,
+			Serial:   serial,
 		})
 	}
 	sort.Slice(gpus, func(i, j int) bool {