Rework the interface into a consistent visual system.

Replace the accumulated per-widget decoration with short, documented
scales for type, weight, radius and motion. Fills are flat and elevation
is neutral, so accent colour marks state instead of decorating: the
gradients on buttons, pills, nodes and progress bars are gone, along with
the coloured glow shadows and the triple ring on the current unit.

Drop draw_node_halo() and the gradient aura on the learning path, which
painted the same bloom in the cairo layer.

Give the welcome page the app icon, a single-colour heading and an
unboxed paragraph. Restore keyboard focus rings, which were suppressed
everywhere, and clamp exercise bodies to a readable measure on wide
windows.

Fix two layout bugs found on the way: the statistics metric cards were
unequal because hexpand only shares leftover space, and a fraction of
zero still painted a stub of accent colour because GTK gives the progress
node a minimum width.

Co-authored-by: Cursor <cursoragent@cursor.com>

maturita.h

@@ -883,9 +883,6 @@ gboolean on_window_key_pressed(GtkEventControllerKey *controller,
                                       GdkModifierType state,
                                       gpointer user_data);
 GtkWidget *build_welcome_page(void);
-void draw_node_halo(cairo_t *cr, double cx, double cy,
-                           double radius, double r, double g, double b,
-                           double alpha);
 void draw_finish_cell(GtkDrawingArea *area, cairo_t *cr,
                              int width, int height, gpointer data);
 void road_point(double t, double *ox, double *oy);