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/gpu_picker.go

@@ -0,0 +1,71 @@
+package webui
+
+// Shared NVIDIA GPU selection picker.
+//
+// Every page that lets the operator pick GPUs (Load, Burn, Benchmark) renders
+// the same row markup: "GPU N — <model> · <memory> MiB · sn: <serial>". That
+// markup lives here once. Pages keep their own selection-note text, multi-GPU
+// mode toggles and CSS-class prefixes, but call beeGpuPicker.render({...}) from
+// their *RenderGPUList wrapper instead of hand-building each <label>.
+//
+// The serial number is rendered monospace, with the digits that differ across
+// the listed GPUs (usually a run at the end, sometimes the middle or start)
+// emphasised so operators can tell cards apart at a glance.
+//
+// gpuPickerCSS is injected once by layoutHead; gpuPickerJS once by renderPage.
+
+const gpuPickerCSS = `.bee-gpu-row{display:flex;align-items:flex-start;gap:8px;padding:6px 0;cursor:pointer;font-size:13px}
+.bee-gpu-row input[type=checkbox]{width:16px;height:16px;margin-top:2px;flex-shrink:0}
+.bee-gpu-sn{font-family:ui-monospace,SFMono-Regular,Menlo,Consolas,monospace;font-size:12px}
+.bee-gpu-sn b{font-weight:700;color:var(--accent)}`
+
+const gpuPickerJS = `window.beeGpuPicker={
+  _esc:function(s){return String(s).replace(/[&<>"]/g,function(c){return {'&':'&amp;','<':'&lt;','>':'&gt;','"':'&quot;'}[c];});},
+  _commonPrefix:function(arr){
+    arr=arr.filter(Boolean);
+    if(arr.length<2)return 0;
+    var n=0,a=arr[0];
+    while(n<a.length&&arr.every(function(s){return s.charAt(n)===a.charAt(n);}))n++;
+    return n;
+  },
+  _commonSuffix:function(arr,capLeft){
+    arr=arr.filter(Boolean);
+    if(arr.length<2)return 0;
+    var minLen=Math.min.apply(null,arr.map(function(s){return s.length;}));
+    var n=0,a=arr[0];
+    while(n<a.length&&capLeft+n<minLen&&arr.every(function(s){return s.charAt(s.length-1-n)===a.charAt(a.length-1-n);}))n++;
+    return n;
+  },
+  _fmtSerial:function(s,p,q){
+    s=String(s);
+    if(p+q>=s.length){p=0;q=0;}
+    if(p===0&&q===0)return this._esc(s); // nothing distinguishing to highlight
+    return this._esc(s.slice(0,p))+'<b>'+this._esc(s.slice(p,s.length-q))+'</b>'+this._esc(s.slice(s.length-q));
+  },
+  row:function(gpu,checkboxClass,onToggle,prefixLen,suffixLen){
+    var mem=gpu.memory_mb>0?' · '+gpu.memory_mb+' MiB':'';
+    var name=this._esc(gpu.name||('GPU '+gpu.index));
+    var sn=gpu.serial?' · <span class="bee-gpu-sn">sn: '+this._fmtSerial(gpu.serial,prefixLen||0,suffixLen||0)+'</span>':'';
+    return '<label class="bee-gpu-row">'
+      +'<input class="'+checkboxClass+'" type="checkbox" value="'+gpu.index+'" checked'
+      +(onToggle?' onchange="'+onToggle+'"':'')+'>'
+      +'<span><strong>GPU '+gpu.index+'</strong> — '+name+mem+sn+'</span>'
+      +'</label>';
+  },
+  render:function(opts){
+    var root=document.getElementById(opts.rootId);
+    if(!root)return;
+    var gpus=opts.gpus||[];
+    if(!gpus.length){
+      root.innerHTML=opts.emptyHTML||'<p style="color:var(--muted);font-size:13px">No NVIDIA GPUs detected.</p>';
+      if(opts.after)opts.after(0);
+      return;
+    }
+    var serials=gpus.map(function(g){return g.serial||'';});
+    var p=this._commonPrefix(serials);
+    var q=this._commonSuffix(serials,p);
+    var self=this;
+    root.innerHTML=gpus.map(function(g){return self.row(g,opts.checkboxClass,opts.onToggle,p,q);}).join('');
+    if(opts.after)opts.after(gpus.length);
+  }
+};`