fix(blossom): handle UTF-8 characters in filenames during upload auth

Replace btoa() with TextEncoder approach to properly encode Unicode
characters in filenames (spaces, international characters, emojis).
The btoa() function only supports Latin1 encoding and would fail when
the auth event content contained non-ASCII characters.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: pablof7z

blossom/src/utils/auth.ts

@@ -84,8 +84,12 @@ export async function createAuthEvent(
  */
 export function addAuthHeaders(headers: Record<string, string>, authEvent: NDKEvent): Record<string, string> {
     // Serialize and base64 encode the event according to BUD-01
+    // Use TextEncoder to properly handle UTF-8 characters (e.g., Unicode in filenames)
     const serializedEvent = JSON.stringify(authEvent.rawEvent());
-    const encodedEvent = btoa(serializedEvent);
+    const encoder = new TextEncoder();
+    const uint8Array = encoder.encode(serializedEvent);
+    const binaryString = String.fromCharCode(...uint8Array);
+    const encodedEvent = btoa(binaryString);
 
     return {
         ...headers,