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>

build_web.sh

@@ -79,7 +79,9 @@ done
 # Linker flags
 # The font lives at resources/fonts/DejaVuSansMono.ttf and is loaded relative to
 # the resources dir at runtime, so the resources@resources preload covers it.
-LDFLAGS="-s USE_GLFW=3 -s ASYNCIFY -s TOTAL_MEMORY=67108864 -s FORCE_FILESYSTEM=1 --preload-file resources@resources --shell-file $SCRIPT_DIR/web_shell.html -s NO_EXIT_RUNTIME=1"
+# INITIAL_MEMORY + ALLOW_MEMORY_GROWTH: the web build now computes the full STFT
+# up front, so the heap must be able to grow for longer recordings.
+LDFLAGS="-s USE_GLFW=3 -s ASYNCIFY -s INITIAL_MEMORY=67108864 -s ALLOW_MEMORY_GROWTH=1 -s FORCE_FILESYSTEM=1 --preload-file resources@resources --shell-file $SCRIPT_DIR/web_shell.html -s NO_EXIT_RUNTIME=1"
 
 if [ "$BUILD_TYPE" = "debug" ]; then
     LDFLAGS="$LDFLAGS -g -O0 -s ASSERTIONS=1"