build: Phase 2 — stop emitting unused .blockmap + clean dist per build

- L83: nsis.differentialPackage: false — the custom updater does full
  downloads and never consumes the differential blockmap, so don't generate it
  (NsisTarget gates blockmap generation on differentialPackage !== false).
- L84: add clean:dist script (fs.rmSync) run at the front of build:win so dist/
  no longer accumulates old-version installers (~3 GB observed). dist/out were
  already gitignored, so this is local-disk hygiene.

Config only; TS gate unaffected. 137 -> 135 open audit items.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

package.json

@@ -19,7 +19,8 @@
     "ui:build": "vite build",
     "ui:preview": "vite preview",
     "build": "electron-vite build",
-    "build:win": "electron-vite build && electron-builder --win",
+    "clean:dist": "node -e \"require('fs').rmSync('dist',{recursive:true,force:true})\"",
+    "build:win": "npm run clean:dist && electron-vite build && electron-builder --win",
     "start": "electron-vite preview",
     "typecheck:node": "tsc --noEmit -p tsconfig.node.json --composite false",
     "typecheck:web": "tsc --noEmit -p tsconfig.web.json --composite false",