feat: white-tone desktop + login crossfade + BUG-031 FAT chain fix + DESK-E texture panics

- BUG-031: fat32_io.h on-demand FAT sector read (FAT beyond 128KB cache walked OOB, font/wallpaper corrupted past first clusters); host unit test diff=0

- material pack: gen_textures.py wired into build.ps1, startMenuLogo.rgba packaged; missing wallpaper/logo/font now fatal (DESK-E02/E03/E04 red fatal screen)

- login->desktop crossfade 16 steps, boot typewriter screen removed

- IDE exit: Esc detaches and returns to desktop page

- white theme: KS_* light palette, g_wf_light taskbar, hardcoded dark fills replaced

- QEMU verified: labels/clock render, wallpaper + start logo loaded, zero spurious panic

CODE/desktop/wm.c

@@ -250,7 +250,7 @@ void draw_window_frame(desktop_window *w) {
     du_fill_rounded_rect(&g_fb, w->x, w->y, w->w, w->h, KS_BG_SECONDARY, 2);
 
     du_fill_rect_gradient(&g_fb, w->x + 1, w->y + 1, w->w - 2, KATE_TITLEBAR_H - 1,
-                          0xFF0A3050u, KS_BG_SECONDARY);
+                          KS_BG_TERTIARY, KS_BG_SECONDARY);
     du_divider_h(&g_fb, w->x, w->y + KATE_TITLEBAR_H, w->w, KS_BORDER);
 
     du_fill_rect(&g_fb, w->x + 6, w->y + KATE_TITLEBAR_H / 2 - 2, 4, 4, KS_ACCENT2);
@@ -258,7 +258,7 @@ void draw_window_frame(desktop_window *w) {
     du_draw_string(&g_fb, w->title,
                    w->x + 14,
                    w->y + (KATE_TITLEBAR_H - (i64)DU_ASCII_LINE_H) / 2 + 1,
-                   KS_ACCENT, 0xFF0A3050u, DU_ASCII_STEP);
+                   KS_ACCENT, KS_BG_SECONDARY, DU_ASCII_STEP);
 
     int bx = w->x + w->w - CLOSE_BTN_OFFSET;
     int by = w->y + (KATE_TITLEBAR_H - CLOSE_BTN_SIZE) / 2;