more intuitive guide vs fullscreen channel switching controls

src/app.c

@@ -349,10 +349,10 @@ static void handle_event(App *app, const SDL_Event *event) {
             app->about_modal_open = !app->about_modal_open;
             break;
         case SDLK_UP:
-            tune_relative(app, 1);
+            tune_relative(app, app->mode == MODE_GUIDE ? -1 : 1);
             break;
         case SDLK_DOWN:
-            tune_relative(app, -1);
+            tune_relative(app, app->mode == MODE_GUIDE ? 1 : -1);
             break;
         case SDLK_LEFT:
             browse_guide_time(app, -GUIDE_BROWSE_STEP_MINUTES);