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/shared/messaging.ts

@@ -25,8 +25,10 @@ export type NexaMessage =
   | { type: 'nexa:wallet-sign-request'; nonce: string }
   | { type: 'nexa:request-wallet-connect' }
   | { type: 'nexa:wallet-disconnect-request' }
-  // Sign-out (popup -> background). See entrypoints/popup/App.tsx.
-  | { type: 'nexa:sign-out' };
+  | { type: 'nexa:switch-account-request' }
+  // Sign-out / switch account (popup -> background). See entrypoints/popup/App.tsx.
+  | { type: 'nexa:sign-out' }
+  | { type: 'nexa:switch-account' };
 
 /** Response shape for 'nexa:wallet-sign-request', returned via sendResponse (not a dispatched NexaMessage). */
 export type WalletSignResult = { signature: string } | { error: string };