feat(web): proper viewport scaling, instant load, and file downloads

Make the Emscripten build behave like the desktop app for loading, window
scaling, and exporting. All changes are #ifdef __EMSCRIPTEN__-gated or no-op on
desktop, so native behavior is unchanged.

Scaling:
- Drop FLAG_WINDOW_HIGHDPI on web. The emscripten-GLFW shim forces a fixed
  pixel canvas style (!important) when HiDPI-aware, overriding the shell's
  100vw/100vh CSS so the canvas can't fill the page; raylib's own resize/window
  callbacks also disagree about dividing by devicePixelRatio, desyncing the
  framebuffer from the reported screen size.
- Sync raylib's window size to window.innerWidth/innerHeight each frame via
  SetWindowSize (guarded against no-op churn). This keeps screen size, GL
  viewport, and projection consistent, so the UI fills the viewport and reflows
  on resize like the desktop window.

Loading:
- Compute the full-resolution STFT synchronously when a file loads instead of
  the desktop overview-then-deferred-high-res path, which relied on many
  main-loop iterations yielding to the browser and appeared to stall partway.
- Allow the wasm heap to grow (INITIAL_MEMORY + ALLOW_MEMORY_GROWTH) so longer
  recordings fit now that everything is computed up front.

Exports:
- Add Platform_OfferFileToUser(): no-op on desktop (file is already on disk);
  on web it reads the just-written file from MEMFS and triggers a browser
  download, then unlinks the temp copy. Wired into PNG and WAV export, which
  now show just the filename in the status message.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/platform_web.c

@@ -10,7 +10,8 @@
 #include <string.h>
 #include <stdarg.h>
 #include <stdio.h>
-#include <errno.h>   /* ENOSYS */
+#include <errno.h>      /* ENOSYS */
+#include <emscripten.h> /* EM_ASM */
 
 /* ── Helpers ─────────────────────────────────────────────────────────── */
 
@@ -64,3 +65,31 @@ const char *Platform_GetTempDir(void) {
      */
     return "/";
 }
+
+void Platform_OfferFileToUser(const char *path) {
+    if (!path) return;
+    /*
+     * The file was just written into the in-memory (MEMFS) filesystem, which
+     * the user can't see. Read it back and hand it to the browser as a
+     * download, then unlink the temporary copy so we don't leak heap memory.
+     */
+    EM_ASM({
+        var p = UTF8ToString($0);
+        try {
+            var data = FS.readFile(p);                 // Uint8Array
+            var name = p.split('/').pop() || 'download';
+            var blob = new Blob([data], { type: 'application/octet-stream' });
+            var url  = URL.createObjectURL(blob);
+            var a = document.createElement('a');
+            a.href = url;
+            a.download = name;
+            document.body.appendChild(a);
+            a.click();
+            document.body.removeChild(a);
+            setTimeout(function() { URL.revokeObjectURL(url); }, 1000);
+            try { FS.unlink(p); } catch (e) {}
+        } catch (e) {
+            console.error('rspektrum: download failed for ' + p + ': ' + e);
+        }
+    }, path);
+}