feat(box3): QR size + dim screen for camera

box3_voice/main/stimulus.c

@@ -25,10 +25,11 @@ esp_err_t stimulus_init(void) {
     s_qr_screen = lv_obj_create(NULL);
     lv_obj_set_style_bg_color(s_qr_screen, lv_color_white(), 0);
 
-    // 220 px QR (BOX-3 LCD is 320x240) — fills the screen height while the
-    // white background still provides the quiet zone quirc needs.
+    // 160 px QR on the 320x240 LCD — leaves ~40 px white top/bottom and
+    // ~80 px sides as the quiet zone quirc needs to lock the finder patterns
+    // (220 px starved the vertical quiet zone and blocked detection).
     s_qr = lv_qrcode_create(s_qr_screen);
-    lv_qrcode_set_size(s_qr, 220);
+    lv_qrcode_set_size(s_qr, 160);
     lv_qrcode_set_dark_color(s_qr, lv_color_black());
     lv_qrcode_set_light_color(s_qr, lv_color_white());
     lv_qrcode_update(s_qr, "zacus", 5);
@@ -50,7 +51,11 @@ static void show_qr(const char *text) {
     lv_label_set_text(s_qr_caption, text);
     lv_screen_load(s_qr_screen);     // bring the QR to the front
     bsp_display_unlock();
-    ESP_LOGI(TAG, "QR shown: %s", text);
+    // Dim the backlight hard: an emissive LCD at full brightness blooms and
+    // crushes QR contrast for the master's camera. ~35% keeps the modules
+    // readable while killing the glare halo.
+    bsp_display_brightness_set(35);
+    ESP_LOGI(TAG, "QR shown (dimmed): %s", text);
 }
 
 // MIDI note -> frequency (A4=69=440 Hz).