feat(desktop): contact requests, auto-update banner, packaging polish (v2.2.0)

Closes the v2.2.0 roadmap. Desktop client is feature-complete and
ready for first installer builds.

Contact request flow (fills a real gap flagged by the user):
  * lib/tx.ts grows buildContactRequestTx / buildAcceptContactTx /
    buildBlockContactTx with canonical bytes matching mobile.
  * lib/api.ts: fetchContactRequests + ContactRequestRaw.
  * New contact modal — sections/contacts/NewContactModal.tsx — resolves
    @username / DC-address / hex pub via resolveAccount, shows identity
    preview (incl. "has encryption key / key not published" hint),
    fee tier picker (5k / 10k / 50k µT), optional 280-char intro,
    balance guard.
  * Requests inbox — sections/contacts/RequestsList.tsx — polled every
    15 s via /relay/contacts, filters pending, Accept submits
    ACCEPT_CONTACT + adds the peer to local contacts with their
    identity.x25519_pub pre-cached, Block submits BLOCK_CONTACT.
  * ContactsList grows a two-tab header (Contacts / Requests with a
    pending-count badge) + "+ New" button next to the filter input.

Auto-update:
  * hooks/useUpdateCheck.ts — polls /api/update-check on mount and
    every 6 hours; loose semver compares the Gitea release tag
    against this build's app.version (from Electron IPC), ignores
    the node's own update_available flag (it compares vs. the node,
    not the desktop).
  * shell/UpdateBanner.tsx — thin strip above the status bar with
    the new tag, Download button (opens the release URL in the
    default browser), and a dismiss-for-this-tag × so once-seen
    updates don't nag.

Packaging — electron-builder config tightened:
  * artifactName pattern includes version + os + arch.
  * Mac: hardenedRuntime on, dmg + zip outputs, social-networking
    category.
  * Windows: NSIS (full installer, per-user or per-machine) +
    portable exe.
  * Linux: AppImage + deb.
  * Strip source maps and test folders from the asar.
  * publish: null — no auto-publisher yet; Gitea releases are
    uploaded manually for now.
  * directories.output = release/, directories.buildResources =
    resources/ so icons land in a predictable place once we add them.

Version bumped to 2.2.0 in package.json. docs/ROADMAP.md marks
v2.2.0 row complete; remaining work (attachments, code signing,
group chats) moved to a post-v2.2.0 bucket.

desktop/package.json

@@ -1,6 +1,6 @@
 {
   "name": "dchain-desktop",
-  "version": "2.2.0-rc1",
+  "version": "2.2.0",
   "description": "DChain desktop client — Electron shell mirroring the mobile app's functionality with a keyboard-first 3-panel layout.",
   "private": true,
   "main": "dist-electron/main.js",
@@ -36,25 +36,40 @@
   "build": {
     "appId": "com.dchain.desktop",
     "productName": "DChain",
+    "copyright": "Copyright © 2026 DChain contributors",
+    "asar": true,
+    "artifactName": "${productName}-${version}-${os}-${arch}.${ext}",
     "files": [
       "dist/**/*",
-      "dist-electron/**/*"
+      "dist-electron/**/*",
+      "!**/*.map",
+      "!**/node_modules/**/test/**",
+      "!**/node_modules/**/tests/**"
     ],
+    "directories": {
+      "output": "release",
+      "buildResources": "resources"
+    },
     "mac": {
-      "target": [
-        "dmg"
-      ]
+      "target": ["dmg", "zip"],
+      "category": "public.app-category.social-networking",
+      "hardenedRuntime": true,
+      "gatekeeperAssess": false
     },
     "win": {
-      "target": [
-        "nsis"
-      ]
+      "target": ["nsis", "portable"]
+    },
+    "nsis": {
+      "oneClick": false,
+      "allowElevation": true,
+      "allowToChangeInstallationDirectory": true,
+      "createDesktopShortcut": true,
+      "createStartMenuShortcut": true
     },
     "linux": {
-      "target": [
-        "AppImage",
-        "deb"
-      ]
-    }
+      "target": ["AppImage", "deb"],
+      "category": "Network"
+    },
+    "publish": null
   }
 }