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/README.md

@@ -0,0 +1,76 @@
+# DChain Desktop
+
+Electron shell for the DChain messenger and social feed.
+
+Same functionality as the mobile client-app, re-imagined with a
+keyboard-first, 3-panel desktop layout:
+
+```
+┌──────────────────────────────────────────────────────────┐
+│  DChain                                                   │ titlebar (drag)
+├──────┬───────────────────┬────────────────────────────────┤
+│ nav  │      list         │             detail             │
+│ 72px │   340px fixed     │            flex 1              │
+├──────┴───────────────────┴────────────────────────────────┤
+│  ● online  ·  node.example:8080  ·  height 10942          │ status bar
+└──────────────────────────────────────────────────────────┘
+```
+
+Sections (left rail): **Messages · Feed · Wallet · Contacts · Settings · Profile**.
+
+## Quick start
+
+```bash
+cd desktop
+npm install
+npm run dev          # concurrently: Vite dev server + Electron
+```
+
+The first boot will show the Welcome screen. Pick Create to generate
+fresh keys, or Import a `node.json` exported from the mobile client.
+
+## Build
+
+```bash
+npm run build        # produces dist/ (renderer) + dist-electron/ (main) + installers
+```
+
+Default installers are built with `electron-builder`: `.dmg` on macOS,
+NSIS `.exe` on Windows, AppImage + `.deb` on Linux. Adjust `build.*` in
+`package.json` for signing / notarisation.
+
+## Layout
+
+- `electron/` — main + preload. TypeScript, compiled to `dist-electron/`
+  by `tsc -p electron/tsconfig.json`.
+- `src/` — renderer. React + Vite. `@/` aliases to `src/`.
+- `src/shell/` — 3-panel chrome.
+- `src/sections/` — one folder per nav section, each exports `{ List, Detail }`.
+- `src/auth/Welcome.tsx` — shown when no key is loaded.
+- `src/lib/` — api, storage, store, types. Mirrors (without React-Native
+  deps) the relevant pieces of `../client-app/lib/`.
+
+## Security model
+
+Master Ed25519 priv lives in the OS keychain via Electron `safeStorage`
+(macOS Keychain / Windows DPAPI / libsecret). A renderer compromise
+cannot read or exfiltrate the key — it always travels through
+`window.dchain.keyfile.*` IPC, which main.ts validates and mediates.
+
+`contextIsolation: true`, `nodeIntegration: false`. CSP in `index.html`
+pins script sources to `'self'` while allowing `connect-src *` so the
+renderer can hit any node the user configures.
+
+## Pairing (v2.2.0-alpha5+)
+
+Desktop will reuse the same 6-digit-code + relay-envelope handshake as
+the mobile client. The scaffold in `src/auth/Welcome.tsx` stubs the
+button until the polling loop lands.
+
+## Multi-device fan-out
+
+When the node is at v2.2.0-alpha1+, `lib/api.ts:fetchDevices` returns
+every linked X25519 pub for a given identity; the sender then encrypts
+one envelope per device. Legacy nodes return an empty array and the
+client falls back to `IdentityInfo.x25519_pub`, preserving the
+pre-multi-device behaviour.