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/shell/StatusBar.tsx

@@ -0,0 +1,75 @@
+// StatusBar — the 28px strip at the bottom. Surfaces the three bits of
+// information an operator tends to want at a glance:
+//   * Connection state to the configured node (poll /api/netstats).
+//   * Current chain height (last successful poll).
+//   * Node URL (short, hover-tooltip for the full thing).
+//
+// Poll interval is 5 seconds — low enough to feel live, cheap enough
+// that even a free-tier node won't notice.
+
+import React, { useEffect, useState } from 'react';
+import { useStore } from '@/lib/store';
+import { getNetStats, getNodeUrl, onNodeUrlChange } from '@/lib/api';
+
+type ConnState = 'online' | 'connecting' | 'offline';
+
+export function StatusBar(): React.ReactElement {
+  const nodeUrl = useStore(s => s.settings.nodeUrl);
+  const [conn,   setConn]   = useState<ConnState>('connecting');
+  const [height, setHeight] = useState<number | null>(null);
+  const [url,    setUrl]    = useState<string>(getNodeUrl());
+
+  useEffect(() => onNodeUrlChange(setUrl), []);
+
+  useEffect(() => {
+    let cancelled = false;
+    const poll = async () => {
+      try {
+        const s = await getNetStats();
+        if (cancelled) return;
+        setConn('online');
+        setHeight(s.total_blocks);
+      } catch {
+        if (cancelled) return;
+        setConn('offline');
+      }
+    };
+    setConn('connecting');
+    poll();
+    const t = setInterval(poll, 5_000);
+    return () => {
+      cancelled = true;
+      clearInterval(t);
+    };
+  }, [nodeUrl]);
+
+  const dot = conn === 'online' ? '#3ba55d' :
+              conn === 'connecting' ? '#f0b35a' :
+              '#f4212e';
+
+  const shortUrl = url
+    .replace(/^https?:\/\//, '')
+    .replace(/\/$/, '');
+
+  return (
+    <footer style={{
+      height: 28, minHeight: 28,
+      background: '#000',
+      borderTop: '1px solid #1f1f1f',
+      display: 'flex', alignItems: 'center', padding: '0 16px', gap: 14,
+      fontSize: 11, color: '#8b8b8b',
+    }}>
+      <span style={{ display: 'inline-flex', alignItems: 'center', gap: 6 }}>
+        <span style={{
+          width: 8, height: 8, borderRadius: 4, background: dot,
+        }} />
+        {conn}
+      </span>
+      <span style={{ opacity: 0.5 }}>·</span>
+      <span title={url}>{shortUrl}</span>
+      <span style={{ opacity: 0.5 }}>·</span>
+      <span>height {height ?? '—'}</span>
+      <div style={{ flex: 1 }} />
+    </footer>
+  );
+}