feat: scroll the sidebar controls with the mouse wheel when they overflow

The left-hand control panel is laid out top-to-bottom with a running y; with
annotations expanded or a short window, the bottom buttons (export, signal
info, about) ran off the bottom and were unreachable. Offset the content by a
sidebarScroll amount, measure the laid-out height, and let the wheel scroll it
(clamped) when the cursor is over the sidebar and no modal is open. No visible
scrollbar; the spectrogram's wheel-zoom already ignores the sidebar column so
the two never both fire.

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

src/spectrogram_types.h

@@ -187,6 +187,9 @@ typedef struct {
     // Overlays
     bool showAbout;  // About / help dialog
 
+    // Sidebar scroll offset (px), for when controls overflow a short window
+    float sidebarScroll;
+
     // File browser state
     bool showFileBrowser;
     char browserPath[512];