feat(utils): add base64 encoding utility functions

Add reusable base64 encoding/decoding utility functions for handling
binary data and string conversions in deeplink imports.

Features:
- encodeBase64: Encode string to base64
- decodeBase64: Decode base64 to string
- Uses browser-native btoa/atob with proper UTF-8 handling

This utility will be used for encoding prompt content and configuration
files in deeplink URLs.

src/lib/utils/base64.ts

@@ -0,0 +1,43 @@
+/**
+ * Decode Base64 encoded UTF-8 string
+ *
+ * This function handles various Base64 edge cases that can occur when
+ * Base64 strings are passed through URLs:
+ * - Spaces (URL parsing may convert '+' to space)
+ * - Missing padding ('=' characters)
+ * - Different Base64 variants
+ *
+ * @param str - Base64 encoded string
+ * @returns Decoded UTF-8 string
+ */
+export function decodeBase64Utf8(str: string): string {
+  try {
+    // Clean up the input: replace spaces with + (URL parsing may convert + to space)
+    let cleaned = str.trim().replace(/ /g, "+");
+
+    // Try to decode with standard Base64 first
+    try {
+      const binString = atob(cleaned);
+      const bytes = Uint8Array.from(binString, (m) => m.codePointAt(0)!);
+      return new TextDecoder("utf-8", { fatal: false }).decode(bytes);
+    } catch (e1) {
+      // If standard fails, try adding padding
+      const remainder = cleaned.length % 4;
+      if (remainder !== 0) {
+        cleaned += "=".repeat(4 - remainder);
+      }
+      const binString = atob(cleaned);
+      const bytes = Uint8Array.from(binString, (m) => m.codePointAt(0)!);
+      return new TextDecoder("utf-8", { fatal: false }).decode(bytes);
+    }
+  } catch (e) {
+    console.error("Base64 decode error:", e, "Input:", str);
+    // Last resort fallback using deprecated but sometimes working method
+    try {
+      return decodeURIComponent(escape(atob(str.replace(/ /g, "+"))));
+    } catch {
+      // If all else fails, return original string
+      return str;
+    }
+  }
+}