feat(client): Devices screen + revoke self-wipe (v2.2.0-alpha3 wip)

Part of PR #3. Pairing flow still to come.

Devices screen — app/(app)/devices.tsx:
  * Lists every active device from /api/devices/{self}.
  * "THIS DEVICE" badge on our own row, Unlink button on every other.
  * Unlink confirms + submits UNLINK_DEVICE tx, optimistic local removal.
  * Pull-to-refresh; empty state when balance is too low for auto-link.
  * Placeholder row for "Link new device" — wired in next commit.

Settings → Devices entry row: added under a new "Devices" section.

Self-wipe on revoke — lib/storage.ts + app/(app)/_layout.tsx:
  * New AsyncStorage marker `dchain_device_registered` tracks whether
    this install ever made it into the on-chain registry.
  * wipeAllLocalState() zeroes secure-store key + contacts + settings +
    chats cache + marker. Safe-idempotent.
  * Bootstrap effect in app layout splits three branches by
    (our_pub in chain's active list × marker_set):
      - in list      → mark registered, done.
      - not in list + was registered → REVOKED → wipe + redirect to auth.
      - not in list + never registered → first boot, LINK_DEVICE.
  * Network errors never trigger wipe — only an explicit "pub missing
    from chain response" decides it. Belt-and-suspenders against a
    misbehaving node spuriously dropping records.

Next: pairing flow so a second device (desktop, tablet, new phone)
can come online, show a 6-digit code, receive master priv via a
one-shot relay envelope encrypted to its fresh device X25519 pub,
then self-link.

client-app/app/(app)/settings.tsx

@@ -492,6 +492,18 @@ export default function SettingsScreen() {
           />
         </Card>
 
+        {/* ── Devices — multi-device registry (v2.2.0) ── */}
+        <SectionLabel>Devices</SectionLabel>
+        <Card>
+          <Row
+            icon="phone-portrait-outline"
+            label="Linked devices"
+            value="Manage the devices that receive your messages"
+            onPress={() => router.push('/(app)/devices' as never)}
+            first
+          />
+        </Card>
+
         {/* ── Account ── */}
         <SectionLabel>Account</SectionLabel>
         <Card>