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/style.css

@@ -79,10 +79,13 @@ header .subtitle {
   opacity: 0.65;
 }
 
+.account-actions {
+  display: flex;
+  gap: 8px;
+}
+
 .sign-out-button {
-  display: block;
-  width: 100%;
-  margin-top: 4px;
+  flex: 1;
   padding: 6px 10px;
   border: 1px solid rgba(128, 128, 128, 0.4);
   border-radius: 6px;