fix: ESLint errors and Prettier formatting

ESLint errors:
- usePresenceUpdater: remove redundant `const userId` inside handlePageHide
  that shadowed the outer declaration (no-shadow)
- RoomViewHeader: prefix unused encryptedRoom with _ (no-unused-vars)

Prettier: reformat 14 files to match project style

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

index.html

@@ -29,10 +29,7 @@
 
     <link rel="preconnect" href="https://fonts.googleapis.com" crossorigin />
     <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin />
-    <link
-      href="https://fonts.googleapis.com/css2?family=VT323&display=swap"
-      rel="stylesheet"
-    />
+    <link href="https://fonts.googleapis.com/css2?family=VT323&display=swap" rel="stylesheet" />
     <link rel="stylesheet" href="/fonts/custom-fonts.css" />
     <link id="favicon" rel="shortcut icon" href="./public/favicon.ico" />