fix: bake UI font atlas at physical DPI size so text isn't skinny on standard-DPI

The font atlas was rasterized once at a fixed 16px and point-filtered, then
scaled to a logical size at draw time. On HiDPI it got upscaled (acceptable);
on standard-DPI it got downscaled, and point filtering dropped rows/columns of
the anti-aliased glyphs, thinning stems into a faint "hyper-skinny" look.

Bake the atlas at the real physical size text is drawn at
(16 * GetUIScale() * GetWindowScaleDPI().y), quantized to 4px steps, and rebuild
it via EnsureUIFont() once per frame when that density changes (resize, or
moving between monitors of different DPI). Use mipmaps + trilinear filtering so
downsampling stays smooth. Call sites are unchanged: DrawTextEx still scales the
atlas glyph to the same logical size, only the backing resolution differs.

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

src/spectrogram.c

@@ -779,10 +779,11 @@ int main(int argc, char* argv[])
 
     SearchAndSetResourceDir("resources");
 
-    // Load TTF font at a fixed base size. Scaling is handled uniformly by
-    // GetUIScale() for both layout and DrawTextScaled(), so the font scales
-    // naturally with the window size on any DPI monitor.
-    mainFont = LoadFontEx("fonts/DejaVuSansMono.ttf", 16, 0, 0);
+    // Load the TTF font. The atlas is rasterized at the physical pixel size text
+    // is actually drawn at (window UI scale * monitor DPI scale) and rebuilt by
+    // EnsureUIFont() when that density changes, so glyphs stay crisp on both
+    // HiDPI and standard-DPI displays instead of thinning out when downscaled.
+    InitUIFont("fonts/DejaVuSansMono.ttf");
     if (mainFont.texture.id == 0) {
         TraceLog(LOG_WARNING, "Failed to load TTF font, using default bitmap font");
     }
@@ -1491,6 +1492,11 @@ int main(int argc, char* argv[])
             app.showAbout = false;
         }
 
+        // Keep the font atlas baked at the current display density (window scale
+        // * DPI). Cheap no-op unless the density actually changed (resize, or
+        // dragging the window between monitors of different DPI).
+        EnsureUIFont();
+
         // Rendering
         BeginDrawing();
         ClearBackground((Color){ 30, 30, 30, 255 });