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/sections/wallet/index.tsx

@@ -0,0 +1,42 @@
+import React, { useEffect, useState } from 'react';
+import { SectionPlaceholder } from '@/shell/SectionPlaceholder';
+import { useStore } from '@/lib/store';
+import { getBalance } from '@/lib/api';
+
+function formatT(ut: number): string {
+  return (ut / 1_000_000).toLocaleString(undefined, { maximumFractionDigits: 3 });
+}
+
+export function WalletList(): React.ReactElement {
+  const keyFile = useStore(s => s.keyFile);
+  const [balance, setBalance] = useState<number | null>(null);
+  useEffect(() => {
+    if (!keyFile) return;
+    getBalance(keyFile.pub_key).then(setBalance).catch(() => setBalance(null));
+  }, [keyFile]);
+  return (
+    <div style={{ padding: 14 }}>
+      <div style={{
+        borderRadius: 14, padding: 14,
+        background: '#0a0a0a', border: '1px solid #1f1f1f',
+      }}>
+        <div style={{ color: '#8b8b8b', fontSize: 11, fontWeight: 700, textTransform: 'uppercase', letterSpacing: 1 }}>
+          Balance
+        </div>
+        <div style={{ color: '#fff', fontSize: 22, fontWeight: 800, marginTop: 4 }}>
+          {balance === null ? '—' : `${formatT(balance)} T`}
+        </div>
+        <div className="selectable" style={{
+          color: '#8b8b8b', fontSize: 11, fontFamily: 'monospace',
+          marginTop: 6, wordBreak: 'break-all',
+        }}>
+          {keyFile?.pub_key}
+        </div>
+      </div>
+    </div>
+  );
+}
+
+export function WalletDetail(): React.ReactElement {
+  return <SectionPlaceholder title="Wallet" note="Transaction history — coming soon." centered />;
+}