fix(gui): make the Settings app actually apply changes

The Settings window rendered fine but its controls only mutated the app's local
settings_data_t -- nothing was ever wired to the running system, so toggling
options did nothing ("not working").

- Appearance: "Show wallpaper grid overlay" now calls gui_desktop_set_grid()
  live; the desktop grid shows/hides immediately.
- Input: "Show seconds in taskbar clock" now calls gui_desktop_set_clock_seconds();
  the taskbar clock switches between HH:MM and HH:MM:SS.
- Power: the CPUFreq governor radios now call cpufreq_set_governor() (mapping the
  UI order to the enum), and Apply/Defaults push all preferences to the system.
- The app seeds its controls from live state on open (grid/clock/governor), so it
  reflects reality instead of hard-coded defaults.

Also add public gui_desktop_set_grid/get_grid and set/get_clock_seconds, and
give Settings a dedicated event handler that frees its state on close (the
shared app_forward_event leaked it).

Verified: Settings renders and its controls are wired to real effects; clean
build on x86_64/aarch64/armv8i; boots to the login gate with 0 panics / 0 OOM.

Co-Authored-By: Claude Opus 4.8 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Fh5z6V7nGPbHvqVG5p4CXX

gui/gui_desktop.c

@@ -78,6 +78,7 @@ static int  g_context_x = 0, g_context_y = 0;
 static bool g_desktop_running   = false;
 static bool g_prev_left = false, g_prev_right = false;
 static bool g_show_grid = true;
+static bool g_clock_seconds = true;
 static int  g_selected_icon = -1;
 static int  g_hover_menu_item = -1;
 static uint64_t g_frames = 0;
@@ -359,10 +360,19 @@ void gui_desktop_render_taskbar(void) {
 
     rtc_time_t now;
     rtc_get_time(&now);
-    snprintf(buf, sizeof(buf), "%02u:%02u:%02u", now.hour, now.minute, now.second);
+    if (g_clock_seconds) {
+        snprintf(buf, sizeof(buf), "%02u:%02u:%02u", now.hour, now.minute, now.second);
+    } else {
+        snprintf(buf, sizeof(buf), "%02u:%02u", now.hour, now.minute);
+    }
     gui_gfx_draw_string_16_shadow(sw - 84, ty + 8, buf, GUI_THEME_PRIMARY, GUI_COLOR_BLACK);
 }
 
+void gui_desktop_set_grid(bool on)          { g_show_grid = on; }
+bool gui_desktop_get_grid(void)             { return g_show_grid; }
+void gui_desktop_set_clock_seconds(bool on) { g_clock_seconds = on; }
+bool gui_desktop_get_clock_seconds(void)    { return g_clock_seconds; }
+
 // ===========================================================================
 // Start menu
 // ===========================================================================