fix: scanner catalog empty + left column blank

Scanner: new instance with existing sf_scan_state.json
did not populate in-memory _catalog. Now re-inspects
unchanged files to fill catalog on first scan.
Frontend: left column reads instrumentSelectorData.
soundfonts instead of sfPresets (null when no presets).
Click SF in left column triggers on-demand fetch.

app/static/js/app.jsx

@@ -17704,15 +17704,29 @@ const App = () => {
           onClick: () => { setSelectedSoundFontId(null); setSynthCategory(null); },
           className: "w-full text-left px-3 py-2 text-sm rounded " + (!selectedSoundFontId ? "bg-amber-700 text-white" : "bg-zinc-800 hover:bg-zinc-700 text-zinc-300")
         }, "All Instruments"),
-        sfPresets === null ? (
-          /*#__PURE__*/React.createElement("p", { className: "text-sm text-zinc-500 py-2" }, "Loading...")
-        ) : (
-          [...new Map(sfPresets.map(p => [p._sfId, p])).values()].map(sf => /*#__PURE__*/React.createElement("button", {
-            key: sf._sfId,
-            onClick: () => setSelectedSoundFontId(sf._sfId),
-            className: "w-full text-left px-3 py-2 text-sm rounded " + (selectedSoundFontId === sf._sfId ? "bg-amber-700 text-white" : "bg-zinc-800 hover:bg-zinc-700 text-zinc-300")
-          }, sf._sfName))
-        )
+        (instrumentSelectorData?.soundfonts || []).map(sf => {
+          const sfId = sf.id.startsWith('sf_') ? sf.id : 'sf_' + sf.id;
+          const sfName = sf.display || sf.name || sf.id;
+          return /*#__PURE__*/React.createElement("button", {
+            key: sfId,
+            onClick: () => {
+              setSelectedSoundFontId(sfId);
+              // Fetch instruments on demand if not yet loaded
+              if (!sf.presets && window.SonicAPI) {
+                const baseId = sfId.replace('sf_', '');
+                window.SonicAPI.listSoundfontInstruments(baseId).then(data => {
+                  if (data && data.presets) {
+                    const mapping = data.presets.map(p => ({ ...p, _sfId: sfId, _sfName: sfName, _sfDisplay: sfName.substring(0, 30) }));
+                    setSfPresets(prev => prev ? [...prev, ...mapping] : mapping);
+                    // Also update instrumentSelectorData cache
+                    setInstrumentSelectorData(prev => prev ? { ...prev, soundfonts: (prev.soundfonts || []).map(s => s.id === sf.id ? { ...s, presets: data.presets } : s) } : prev);
+                  }
+                }).catch(() => {});
+              }
+            },
+            className: "w-full text-left px-3 py-2 text-sm rounded " + (selectedSoundFontId === sfId ? "bg-amber-700 text-white" : "bg-zinc-800 hover:bg-zinc-700 text-zinc-300")
+          }, sfName);
+        })
       ),
       /*#__PURE__*/React.createElement("div", { className: "flex-1 overflow-y-auto space-y-0.5" },
         /*#__PURE__*/React.createElement("button", {