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>

i18n.c

@@ -1454,20 +1454,11 @@ void i18n_bind(GtkWidget *widget, const char *key, int kind) {
 }
 
 void refresh_welcome_heading(void) {
-    char *markup;
-
     if (!welcome_heading)
         return;
-    if (app_lang == LANG_EN)
-        markup = g_strdup_printf(
-            "Welcome to <span color=\"#%06x\">maturita.C</span>!",
-            app_theme.accent);
-    else
-        markup = g_strdup_printf(
-            "Vítejte ve <span color=\"#%06x\">maturita.C</span>!",
-            app_theme.accent);
-    gtk_label_set_markup(GTK_LABEL(welcome_heading), markup);
-    g_free(markup);
+    gtk_label_set_text(GTK_LABEL(welcome_heading),
+                       app_lang == LANG_EN ? "Welcome to maturita.C"
+                                           : "Vítejte v maturita.C");
 }
 
 void refresh_stats_ui(void);