fix: add precons.js with 163 entries and fix backend precon UI refresh (#197)

- Generate lan-client/precons.js from precon-decks/precon-index.json so
  PRECON_DATA is always available for local/offline mode (163 precons)
- Add scripts/generate-precons-js.js for regeneration when precons update
- Fix _loadBackendPrecons() to re-render the precon tab UI when backend
  precons load successfully (was only updating status indicator text,
  not populating the dropdown)

Without precons.js, PRECON_DATA was undefined and the dropdown showed
'No precon data loaded' even though 163 .dck files exist in the repo.

scripts/generate-precons-js.js

@@ -0,0 +1,35 @@
+#!/usr/bin/env node
+/**
+ * Generate lan-client/precons.js from precon-decks/precon-index.json
+ * Usage: node scripts/generate-precons-js.js
+ */
+const fs = require('fs');
+const path = require('path');
+
+const indexPath = path.join(__dirname, '..', 'precon-decks', 'precon-index.json');
+const outputPath = path.join(__dirname, '..', 'lan-client', 'precons.js');
+
+const index = JSON.parse(fs.readFileSync(indexPath, 'utf8'));
+
+let js = '// Auto-generated from precon-decks/precon-index.json\n';
+js += '// Run: node scripts/generate-precons-js.js to regenerate\n';
+js += 'var PRECON_DATA = [\n';
+
+index.forEach(function(p, i) {
+  const entry = {
+    id: (p.fileName || p.name || '').replace('.dck', ''),
+    name: p.name || '',
+    commander: p.commander || (p.commanders && p.commanders[0]) || '',
+    set: p.set || '',
+    fileName: p.fileName || '',
+    colors: p.colors || [],
+    year: p.year || 0,
+  };
+  js += '  ' + JSON.stringify(entry);
+  js += (i < index.length - 1) ? ',\n' : '\n';
+});
+
+js += '];\n';
+
+fs.writeFileSync(outputPath, js);
+console.log(`Generated ${outputPath} with ${index.length} precon entries`);