feat: Hull/Fill nodes freely addable+deletable; remove Fill view tab

NodeGraph:
  isFixed now only applies to Source — Hull and Fill are fully deletable,
  supporting arbitrary multi-pass branching (multiple Hull/Fill chains from
  the same detection graph).
  addNode() handles Hull (defaultHullParams) and Fill (defaultFillParams).
  Toolbar gains teal '+ Hull' and purple '+ Fill' buttons alongside the
  existing Kernel/Combine buttons.

App.jsx:
  Remove 'fill' from VIEW_MODES — fill output is visible in the detection
  graph's Fill node thumbnail and in the G-code view.
  Remove the dead case 'fill': branch from the viewport refresh switch.

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

src-frontend/src/App.jsx

@@ -8,7 +8,7 @@ import { defaultPass, defaultGcodeConfig, PAPER_SIZES } from './store.js'
 import * as tauri from './hooks/useTauri.js'
 import { useFps } from './hooks/useFps.js'
 
-const VIEW_MODES = ['source', 'detection', 'contours', 'fill', 'gcode']
+const VIEW_MODES = ['source', 'detection', 'contours', 'gcode']
 
 export default function App() {
   const [image, setImage]           = useState(null)
@@ -101,10 +101,6 @@ export default function App() {
             setDisplayB64(null)
           }
           break
-        case 'fill':
-          // Fill uses the same full-res canvas rendering as gcode (via strokes prop).
-          setDisplayB64(null)
-          break
         case 'gcode':
           if (passes.some(p => p.strokeCount > 0)) {
             try {

src-frontend/src/components/NodeGraph.jsx

@@ -189,8 +189,9 @@ export default function NodeGraph({ graph, onChange, nodePreviews, sourceImageB6
     const x = (r.width  / 2 - p.x) / z - NODE_W / 2
     const y = (r.height / 2 - p.y) / z - 60
     const id = newNodeId(kind)
-    const node = kind === 'Kernel'
-      ? { id, kind, x, y, ...defaultKernelProps() }
+    const node = kind === 'Kernel'  ? { id, kind, x, y, ...defaultKernelProps() }
+               : kind === 'Hull'    ? { id, kind, x, y, ...defaultHullParams() }
+               : kind === 'Fill'    ? { id, kind, x, y, ...defaultFillParams() }
                : { id, kind, x, y, blend_mode: 'Average', inputCount: 2 }
     const g = graphRef.current
     onChangeRef.current({ ...g, nodes: [...g.nodes, node] })
@@ -222,7 +223,7 @@ export default function NodeGraph({ graph, onChange, nodePreviews, sourceImageB6
 
   // ── Node rendering ─────────────────────────────────────────────────────────
   function renderNode(node) {
-    const isFixed  = node.kind === 'Source' || node.kind === 'Hull' || node.kind === 'Fill'
+    const isFixed  = node.kind === 'Source'
     const inputCnt = node.kind === 'Combine' ? (node.inputCount ?? 2)
                    : (node.kind === 'Kernel' || node.kind === 'Output' || node.kind === 'Hull' || node.kind === 'Fill') ? 1 : 0
     const hasOut   = node.kind !== 'Output' && node.kind !== 'Hull' && node.kind !== 'Fill'
@@ -440,12 +441,16 @@ export default function NodeGraph({ graph, onChange, nodePreviews, sourceImageB6
     >
       {/* Toolbar */}
       <div style={{ position: 'absolute', top: 8, left: 8, zIndex: 30, display: 'flex', gap: 4, alignItems: 'center' }}>
-        <button onClick={() => addNode('Kernel')}
-          style={{ padding: '3px 10px', borderRadius: 4, fontSize: 11, cursor: 'pointer', border: '1px solid #374151', background: '#1e293b', color: '#94a3b8' }}
-        >+ Kernel</button>
-        <button onClick={() => addNode('Combine')}
-          style={{ padding: '3px 10px', borderRadius: 4, fontSize: 11, cursor: 'pointer', border: '1px solid #374151', background: '#1e293b', color: '#94a3b8' }}
-        >+ Combine</button>
+        {[
+          ['Kernel',  '#374151', '#94a3b8'],
+          ['Combine', '#374151', '#94a3b8'],
+          ['Hull',    '#0d9488', '#5eead4'],
+          ['Fill',    '#7c3aed', '#c4b5fd'],
+        ].map(([kind, border, color]) => (
+          <button key={kind} onClick={() => addNode(kind)}
+            style={{ padding: '3px 10px', borderRadius: 4, fontSize: 11, cursor: 'pointer', border: `1px solid ${border}`, background: '#1e293b', color }}
+          >+ {kind}</button>
+        ))}
         <span style={{ fontSize: 10, color: '#2d3748', paddingLeft: 4 }}>
           scroll=zoom · drag=pan · click wire=delete
         </span>