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/ui.c

@@ -318,8 +318,9 @@ void ExportPNG(const SpectrogramApp* spa, const char* dirPath)
     }
 
     if (ExportImage(region, path)) {
+        Platform_OfferFileToUser(path);  // desktop: no-op; web: browser download
         snprintf((char*)spa->exportMessage, sizeof(spa->exportMessage),
-                 "Exported: %dx%d %.40s", region.width, region.height, path);
+                 "Exported: %dx%d %.40s", region.width, region.height, GetFileName(path));
     } else {
         snprintf((char*)spa->exportMessage, sizeof(spa->exportMessage), "Export failed");
     }