build: document system deps, add check-deps, auto-glob web modules

- README: rewrite Build section for the hand-written Makefile (make /
  make DEBUG=1 / run / test / bench). Add a "System dependencies" table
  with the X11+GL dev packages per distro (apt/dnf/pacman/zypper/apk),
  since a clone won't build without them and the error is otherwise
  cryptic. Fix stale ./bin/Debug paths to ./bin/Release.
- Makefile: add `make check-deps` — probes for the required X11/GL dev
  headers and prints the install command for the detected distro on
  failure (via /etc/os-release).
- build_web.sh: auto-discover app modules from src/*.c (drop the
  hardcoded APP_MODULES list that had to be hand-synced), matching the
  desktop Makefile. Excludes the desktop platform backends. Remove the
  stale rspektrum.make reference.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

build_web.sh

@@ -50,12 +50,21 @@ fi
 
 # Compile the application modules (web version — no subprocess support).
 # platform_web.c provides stub implementations for spawn/error functions.
-# Keep this list in sync with rspektrum.make (use platform_web instead of
-# platform_linux for the web target).
+#
+# Auto-discovered from src/*.c (same as the desktop Makefile) so adding a module
+# needs no edit here. The web target keeps platform_web.c and drops the desktop
+# backends (platform_linux.c / platform_win32.c).
 echo "=== Step 2: Compiling spectrogram modules for web ==="
 cd "$SCRIPT_DIR"
 
-APP_MODULES="spectrogram fft stft audio render ui utils primitives mlnl platform_web"
+APP_MODULES=""
+for f in "$SRC_DIR"/*.c; do
+    m=$(basename "$f" .c)
+    case "$m" in
+        platform_linux|platform_win32) continue ;;  # desktop-only backends
+    esac
+    APP_MODULES="$APP_MODULES $m"
+done
 
 APP_OPT="-Os"
 if [ "$BUILD_TYPE" = "debug" ]; then