Fix duplicate signing; make account switching an explicit action

Root cause of "requires a lot of signs, not just a single one":
inject.ts wired Phantom's own accountChanged provider event to
auto-trigger re-auth, but calling connect() ourselves also fires that
same event -- so a normal silent reconnect raced its own
event-triggered handler, producing two competing "wallet connected"
reports that each independently asked Phantom to sign a fresh nonce.

Removed that event wiring entirely (onWalletEvent, EVENT_CHANNEL) --
inject.ts now only responds to our own explicit calls, never reacts
to unsolicited provider events. Per feedback, account switching isn't
something that should be inferred from a Phantom event anyway; it's
now its own explicit feature: a "Switch account" button in the popup
(nexa:switch-account) that tells the content script to disconnect and
immediately reconnect, so Phantom's connect UI reflects whichever
account is currently active there.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01YXiHuScXrjxBh7yFGAPq3B

src/entrypoints/background.ts

@@ -1,7 +1,12 @@
 import { clearSessionToken, getSessionToken, getStoredSession } from '@/background/backend-client';
 import { getConnectionStatus, setConnectionStatus } from '@/background/connection-status';
 import { applyLockState, getLockState } from '@/background/lock-state';
-import { handleWalletConnected, requestWalletDisconnect, requestWalletReconnect } from '@/background/wallet-auth';
+import {
+  handleWalletConnected,
+  requestAccountSwitch,
+  requestWalletDisconnect,
+  requestWalletReconnect,
+} from '@/background/wallet-auth';
 import { connectWsClient } from '@/background/ws-client';
 import type { NexaMessage } from '@/shared/messaging';
 
@@ -39,12 +44,11 @@ export default defineBackground(() => {
 
       case 'nexa:wallet-connected':
         // A content script reports this on every page load (it always tries
-        // a silent onlyIfTrusted connect first), AND whenever the user
-        // switches accounts in Phantom's own UI. Only re-authenticate (which
-        // means asking Phantom to sign a fresh nonce — a popup every time,
-        // unlike a silent connect) when this isn't the wallet we're already
-        // signed in as; a bare "do we have a token" check can't tell those
-        // apart from an account switch.
+        // a silent onlyIfTrusted connect first), and after an explicit
+        // account switch. Only re-authenticate (which means asking Phantom
+        // to sign a fresh nonce — a popup every time, unlike a silent
+        // connect) when this isn't the wallet we're already signed in as; a
+        // bare "do we have a token" check can't tell those apart.
         getStoredSession()
           .then((session) => {
             if (session?.walletAddress === message.walletAddress) return;
@@ -63,6 +67,10 @@ export default defineBackground(() => {
         void signOut();
         return false;
 
+      case 'nexa:switch-account':
+        void requestAccountSwitch();
+        return false;
+
       default:
         return undefined;
     }