Add subscription debug panel (Ctrl+Shift+D)

Hidden dev tool showing NDK uptime, live subscription status,
per-relay connection state, per-tab last-updated timestamps,
and recent feed diagnostics log. Polls every 2s while visible.
Closes with Ctrl+Shift+D, Escape, or X button.

src/lib/nostr/core.ts

@@ -57,16 +57,22 @@ export function saveRelayUrls(urls: string[]) {
 }
 
 let ndk: NDK | null = null;
+let ndkCreatedAt: number | null = null;
 
 export function getNDK(): NDK {
   if (!ndk) {
     ndk = new NDK({
       explicitRelayUrls: getStoredRelayUrls(),
     });
+    ndkCreatedAt = Date.now();
   }
   return ndk;
 }
 
+export function getNDKUptimeMs(): number | null {
+  return ndkCreatedAt ? Date.now() - ndkCreatedAt : null;
+}
+
 /**
  * Destroy the current NDK instance and create a fresh one.
  * Preserves the signer (login state) but resets all relay connections.
@@ -87,6 +93,7 @@ export async function resetNDK(): Promise<void> {
   ndk = new NDK({
     explicitRelayUrls: getStoredRelayUrls(),
   });
+  ndkCreatedAt = Date.now();
 
   // Restore signer so user stays logged in
   if (oldSigner) {