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/popup/App.tsx

@@ -38,6 +38,10 @@ export function App() {
     void browser.runtime.sendMessage({ type: 'nexa:sign-out' } satisfies NexaMessage);
   }
 
+  function switchAccount(): void {
+    void browser.runtime.sendMessage({ type: 'nexa:switch-account' } satisfies NexaMessage);
+  }
+
   return (
     <>
       <header>
@@ -64,9 +68,14 @@ export function App() {
       </section>
 
       {connectionStatus === 'connected' && (
-        <button type="button" className="sign-out-button" onClick={signOut}>
-          Sign out
-        </button>
+        <div className="account-actions">
+          <button type="button" className="sign-out-button" onClick={switchAccount}>
+            Switch account
+          </button>
+          <button type="button" className="sign-out-button" onClick={signOut}>
+            Sign out
+          </button>
+        </div>
       )}
 
       <a