Read battery once, after the picture is pushed, not at boot

battery_read_percent() was called once at the very start of boot, before
WiFi even connects, and that value was reused both for the manage-menu
overlay and the server report. Taken right after a reset (e.g. the OTA
reboot that immediately precedes it), the rail may still be settling --
plausible source of noisy jumps in reported battery level.

Now there's a single read, in frame_client_run() right before
report_battery(), after the photo (and manage overlay, if shown) is
already on the panel -- the fetch/display work already done this cycle
is the settle time, no delay to guess. The manage overlay no longer
needs an early local reading at all: it shows the server's last-known
value instead, added to the /frame/photo-info response it already
fetches.

firmware/main/main.c

@@ -10,7 +10,6 @@
 #include "next_button.h"
 #include "back_button.h"
 #include "combo_button.h"
-#include "battery.h"
 
 static const char *TAG = "main";
 
@@ -52,18 +51,12 @@ void app_main(void)
      * for "not pressed" (false) or "quick press" (true, show the menu). */
     bool show_management_qr = combo_button_check();
 
-    /* Must come after the button checks: the battery pin is (by design,
-     * on the XIAO board) shared with a button, and the ADC read briefly
-     * takes the pin over -- see battery.h. -1 = no reading (disabled,
-     * on mains, or implausible). */
-    int battery_percent = battery_read_percent();
-
     frame_config_t cfg;
     esp_err_t cfg_err = frame_config_load(&cfg);
     if (cfg_err == ESP_OK) {
         ESP_LOGI(TAG, "Found stored config for '%s', connecting to home WiFi", cfg.sta_ssid);
         if (frame_wifi_connect_sta(&cfg) == ESP_OK) {
-            frame_client_run(&cfg, action, show_management_qr, battery_percent);
+            frame_client_run(&cfg, action, show_management_qr);
             return; /* frame_client_run currently never returns */
         }
         ESP_LOGW(TAG, "Could not connect to stored WiFi after %d attempts, falling back to provisioning",