fix(desktop): CSP via webRequest + boot error visibility

Two problems from the first alpha4 run reported as "blank window + CSP
warning in devtools":

1. CSP was set via <meta> in index.html with a strict policy (script-src
   'self'). Vite's dev server uses eval() for HMR, which the strict CSP
   blocked at module-load time, so the renderer never ran. The meta CSP
   also conflicted with Electron's own security heuristics (hence the
   warning even though *we* had a policy — Electron was looking for it
   on the HTTP response).

   Moved the CSP to electron/main.ts via session.webRequest
   .onHeadersReceived. Dev profile enables 'unsafe-eval' + ws:/wss: for
   HMR; production profile stays strict (no eval, no remote scripts,
   connect-src still wide because the user picks arbitrary node URLs).

2. When window.dchain isn't available (preload failed to load, dev
   misconfig, etc.), loadKeyFile() throws inside a useEffect. React
   swallows async-effect throws, so the app renders blank forever.

   Added:
     - requireDchain() guard in storage.ts with an explicit error.
     - App.tsx catches boot-effect errors and renders them inline.
     - ErrorBoundary.tsx for render-time throws.
     - window.addEventListener('error') in main.tsx as a last-resort
       paint for throws that escape React entirely.

Also: npm script electron:dev now rebuilds main.ts before spawning
Electron (was a silent concurrency bug — TypeScript errors in main.ts
would produce stale dist-electron/).

desktop/package.json

@@ -6,8 +6,8 @@
   "main": "dist-electron/main.js",
   "scripts": {
     "dev": "concurrently -k -n vite,electron -c blue,magenta \"vite\" \"wait-on tcp:5173 && npm run electron:dev\"",
-    "electron:dev": "tsc -p electron/tsconfig.json && cross-env VITE_DEV_SERVER_URL=http://localhost:5173 electron dist-electron/main.js",
-    "build": "tsc -p electron/tsconfig.json && vite build && electron-builder",
+    "electron:dev": "npm run build:main && cross-env VITE_DEV_SERVER_URL=http://localhost:5173 electron dist-electron/main.js",
+    "build": "npm run build:main && vite build && electron-builder",
     "build:renderer": "vite build",
     "build:main": "tsc -p electron/tsconfig.json",
     "typecheck": "tsc --noEmit -p tsconfig.json && tsc --noEmit -p electron/tsconfig.json"