build/docs: bump build to 308, point download to v1.1-beta.21

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: mkschulze

docs/download.md

@@ -14,8 +14,8 @@ The new JUCE-based version of JamWide is available as a beta. This is a complete
 **This is beta software** — it may contain bugs. Please report issues on [GitHub](https://github.com/mkschulze/JamWide/issues).
 
 <div class="download-section">
-  <a href="https://github.com/mkschulze/JamWide/releases/tag/v1.1-beta.20" class="btn btn-primary btn-large">
-    Download v1.1-beta.20
+  <a href="https://github.com/mkschulze/JamWide/releases/tag/v1.1-beta.21" class="btn btn-primary btn-large">
+    Download v1.1-beta.21
   </a>
   <p class="version-info">macOS (Universal) / Windows (x64) / Linux (x64)</p>
 </div>

src/build_number.h

@@ -1,2 +1,2 @@
 #pragma once
-#define JAMWIDE_BUILD_NUMBER 303
+#define JAMWIDE_BUILD_NUMBER 308