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/spectrogram.c

@@ -20,6 +20,23 @@
 #include <stdbool.h>
 #include <stdio.h>
 
+#ifdef __EMSCRIPTEN__
+#include <emscripten/emscripten.h>
+
+// Keep raylib's framebuffer/screen size matched to the browser viewport, so the
+// (immediate-mode) UI fills the page and reflows on window resize the same way
+// the desktop OS window does. Going through SetWindowSize keeps the screen size,
+// GL viewport, and projection consistent; the != guard avoids per-frame churn.
+static void SyncCanvasToWindow(void)
+{
+    int w = EM_ASM_INT({ return window.innerWidth;  });
+    int h = EM_ASM_INT({ return window.innerHeight; });
+    if (w > 0 && h > 0 && (w != GetScreenWidth() || h != GetScreenHeight())) {
+        SetWindowSize(w, h);
+    }
+}
+#endif
+
 // ============================================================================
 // Global State (declared extern in spectrogram_types.h)
 // ============================================================================
@@ -217,7 +234,18 @@ static void DispatchKeymap(void)
 
 int main(int argc, char* argv[])
 {
+#ifdef __EMSCRIPTEN__
+    // FLAG_WINDOW_HIGHDPI is buggy on the web backend: the Emscripten resize
+    // callback sets the screen size to window.innerWidth, but the GLFW window-
+    // size callback it triggers divides that by devicePixelRatio when HIGHDPI
+    // is set. On a HiDPI display the framebuffer and the reported screen size
+    // desync and the UI renders into a corner. UI scaling is handled by
+    // GetUIScale() regardless, so the flag is unnecessary here. raylib auto-
+    // resizes the canvas to the window when FLAG_WINDOW_RESIZABLE is set.
+    SetConfigFlags(FLAG_VSYNC_HINT | FLAG_WINDOW_RESIZABLE);
+#else
     SetConfigFlags(FLAG_VSYNC_HINT | FLAG_WINDOW_RESIZABLE | FLAG_WINDOW_HIGHDPI);
+#endif
     InitWindow(1280, 800, "Spectrogram Viewer");
     SetTargetFPS(60);
     SetTraceLogLevel(LOG_WARNING);  // Suppress INFO texture logs
@@ -320,6 +348,11 @@ int main(int argc, char* argv[])
 
     while (!WindowShouldClose())
     {
+#ifdef __EMSCRIPTEN__
+        // Track the browser viewport (fill + reflow on resize, like desktop).
+        SyncCanvasToWindow();
+#endif
+
         // Drag & Drop
         if (IsFileDropped()) {
             FilePathList dropped = LoadDroppedFiles();
@@ -764,6 +797,27 @@ int main(int argc, char* argv[])
 
         // Processing (incremental across frames)
         if (app.loaded && !app.stftComputed) {
+#ifdef __EMSCRIPTEN__
+            // Web build: there are no worker threads, and the desktop path's
+            // overview-then-deferred-high-res fill depends on many main-loop
+            // iterations yielding to the browser (which made loading appear to
+            // stall partway). Compute the full-resolution STFT in one shot so
+            // the spectrogram is completely ready as soon as the file loads.
+            ComputeSTFTInit(&app.signal, &app.stft, app.fftSize);
+            app.skipFactor = 1;  // full resolution, no overview stride
+            ComputeSTFTIncremental(&app.signal, &app.stft, app.fftSize, 0); // computes every segment
+            AutoScaleAmplitude(&app.stft);
+            GenerateSpectrogramTexture(&app.stft, &app.spectrogramImage, &app.spectrogramTexture);
+            app.currentSTFTSegment = app.stft.numSegments;
+            app.bgHighResSeg = app.stft.numSegments;
+            app.loadingProgress = 1.0f;
+            app.stftComputed = true;
+            app.highResFinished = true;
+            app.bgFinished = true;
+            app.isBgProcessing = false;
+            app.loadingPhase = 0;
+            SaveToCache();
+#else
             if (app.loadingPhase == 0) {
                 // Initialize STFT once
                 ComputeSTFTInit(&app.signal, &app.stft, app.fftSize);
@@ -802,6 +856,7 @@ int main(int argc, char* argv[])
                 // Save the overview result to cache (will be overwritten when full-res completes)
                 SaveToCache();
             }
+#endif // __EMSCRIPTEN__
         }
 
         // Loading overlay (drawn during STFT computation)