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/webui/pages.go

@@ -126,6 +126,7 @@ function openComponentDetail(type) {
     body.innerHTML = '<div style="padding:20px;color:var(--crit-fg)">Error loading details.</div>';
   });
 }
+` + gpuPickerJS + `
 </script>` +
 		`</body></html>`
 }