feat(desktop): Electron scaffold, shell, auth + section stubs (v2.2.0-alpha4)

PR #4 of the multi-device roadmap — desktop client groundwork. The shell
compiles and runs end-to-end on top of a v2.2.0 node; sections are
placeholders that later alphas fill in with real chat / feed / wallet /
contacts / settings content shared with the mobile client-app.

Scaffold:
  * Vite + React + TypeScript renderer; Electron main/preload TS
    compiled via a separate tsconfig.
  * npm scripts — `dev` (concurrent Vite + Electron), `build`
    (installer via electron-builder), `typecheck`.
  * electron-builder targets: .dmg / .exe / .AppImage + .deb.
  * CSP pins script-src 'self'; connect-src left open so the renderer
    can hit any configured node.

Electron main + preload:
  * Frame-less window, hiddenInset on macOS, custom-overlay on Windows,
    drag region via CSS -webkit-app-region: drag on our TitleBar.
  * contextIsolation on, nodeIntegration off, sandbox off (needed for
    safeStorage in preload).
  * window.dchain.keyfile.{load,save,delete,encryptionAvailable} —
    keyfile lives in the OS keychain via Electron safeStorage, with a
    plaintext fallback for OSes without an encryption backend.
  * window.dchain.dialog.{openFile,saveFile}, .fs.{readText,writeText},
    .app.{version,platform}. Everything else still goes over plain
    fetch() in the renderer.

Shell (src/shell/):
  * TitleBar — draggable 32px strip; DChain brand.
  * NavBar — left 72px rail, six sections + Cmd+1..5 keybinds.
  * StatusBar — ● online/connecting/offline dot, node URL, current
    chain height (polls /api/netstats every 5s).
  * Shell — composes the 3 panes; picks { List, Detail } by active
    section.

Sections (all stubs — filling in alpha5+):
  * Messages, Feed, Contacts, Profile — SectionPlaceholder with notes.
  * Wallet — shows the balance reading from /api/address/{pub} as a
    first real data binding.
  * Settings — node-URL card with live ping + commit, identity card
    (shows pub key), about card (reads Electron app.version via IPC).

Auth (src/auth/Welcome.tsx):
  * Create — generates Ed25519 + X25519 via tweetnacl, saves via IPC.
  * Import — Electron dialog.openFile → parses node.json → saves.
  * Pair — stub routed; real poll loop reuses the mobile flow in
    alpha5.

Lib (src/lib/):
  * types.ts — KeyFile / Contact / Message / NodeSettings mirroring
    client-app wire formats.
  * storage.ts — keyfile via IPC, settings + contacts + device-registered
    marker via localStorage.
  * api.ts — fetch wrapper with setNodeUrl + onNodeUrlChange;
    getNetStats, getIdentity, fetchDevices, getBalance bindings.
  * store.ts — zustand { booted, keyFile, settings, contacts, section }.

docs/ROADMAP.md — desktop subsection updated with per-alpha breakdown.

Next (alpha5): Messages section wired to the relay mailbox, full
conversation view, and the pairing poll loop.

desktop/src/App.tsx

@@ -0,0 +1,41 @@
+// Top-level component. Two responsibilities:
+//   1. Boot — load key + settings from storage, wire up the API client,
+//      flip the booted flag so we stop showing the black splash.
+//   2. Render either the Welcome auth flow (no key yet) or the Shell
+//      (3-panel layout + current section).
+
+import React, { useEffect } from 'react';
+import { useStore } from '@/lib/store';
+import { loadKeyFile, loadSettings, loadContacts } from '@/lib/storage';
+import { setNodeUrl } from '@/lib/api';
+import { Shell } from '@/shell/Shell';
+import { Welcome } from '@/auth/Welcome';
+
+export function App(): React.ReactElement {
+  const booted   = useStore(s => s.booted);
+  const keyFile  = useStore(s => s.keyFile);
+
+  useEffect(() => {
+    (async () => {
+      const set = loadSettings();
+      setNodeUrl(set.nodeUrl);
+      useStore.getState().setSettings(set);
+
+      const cs = loadContacts();
+      useStore.getState().setContacts(cs);
+
+      const kf = await loadKeyFile();
+      useStore.getState().setKeyFile(kf);
+
+      useStore.getState().setBooted(true);
+    })();
+  }, []);
+
+  if (!booted) {
+    // Matches the splash: whole window is already black from index.html,
+    // so showing nothing is the right behaviour — no flash, no spinner.
+    return <div style={{ height: '100%' }} />;
+  }
+
+  return keyFile ? <Shell /> : <Welcome />;
+}