Fetch refresh interval from server instead of hardcoding it

Replaces check_server_reachable() (bare bool, GET /health) with
fetch_frame_config(), which GETs the server's new /frame/config endpoint
instead -- doubles as the reachability check (any completed HTTP response
counts, same as before) and delivers the server-configured
refresh_interval_s, used for the success-path deep sleep duration instead
of the Kconfig-only default.

Parses the tiny JSON response with a hand-rolled scalar extractor
(json_extract_uint) rather than pulling in a JSON library -- cJSON isn't
bundled in this ESP-IDF install, and a single flat integer field doesn't
justify a new dependency. Verified standalone against exactly the JSON
shape the server emits, including a missing-field fallback case.

FRAME_SLEEP_INTERVAL_S (Kconfig) is now just the fallback used before the
device has ever reached a configured server, or if the response is
missing/unparseable -- documented as such in its help text.

firmware/main/Kconfig.projbuild

@@ -45,11 +45,13 @@ menu "ESPresso Frame Configuration"
             stored home WiFi network before counting it as a failed retry.
 
     config FRAME_SERVER_CHECK_TIMEOUT_MS
-        int "Tools server reachability check timeout (ms)"
+        int "Tools server /frame/config request timeout (ms)"
         default 3000
         help
-            How long to wait for an HTTP response from the tools server
-            when checking reachability on the post-connect status screen.
+            How long to wait for a GET /frame/config response from the
+            tools server -- doubles as both the reachability check for the
+            post-connect status screen and the source of the
+            server-configured refresh interval below.
 
     config FRAME_FETCH_TIMEOUT_MS
         int "Image fetch HTTP timeout (ms)"
@@ -61,12 +63,17 @@ menu "ESPresso Frame Configuration"
             check's timeout.
 
     config FRAME_SLEEP_INTERVAL_S
-        int "Deep sleep interval between refreshes (seconds)"
+        int "Fallback deep sleep interval between refreshes (seconds)"
         default 3600
         help
-            How long the device deep-sleeps between successful
-            fetch-and-display cycles. Lower this for bench testing so you
-            don't have to wait an hour per iteration.
+            The refresh interval is normally set server-side (the "Refresh
+            interval" field in the server's web UI, delivered via GET
+            /frame/config) so it can be changed without reflashing. This
+            value is only a fallback: used before the device has ever
+            successfully reached a configured server, or if the server's
+            response doesn't include a valid interval (e.g. an older
+            server version). Lower this for bench testing so you don't
+            have to wait an hour per iteration.
 
     config FRAME_RETRY_INTERVAL_S
         int "Deep sleep interval after a failed cycle (seconds)"

firmware/main/frame_client.c

@@ -111,12 +111,52 @@ esp_err_t frame_wifi_connect_sta(const frame_config_t *cfg)
     return result;
 }
 
-/* Probes the server's /health endpoint -- any completed HTTP response
- * means the socket-level connection succeeded, which is all this checks. */
-static bool check_server_reachable(const char *toolsserver)
+typedef struct {
+    bool reachable;
+    uint32_t refresh_interval_s; /* CONFIG_FRAME_SLEEP_INTERVAL_S if absent/unparseable */
+} frame_server_config_t;
+
+/* Finds the first integer value associated with "key" in a small JSON
+ * blob, e.g. 3600 in {"refresh_interval_s": 3600}. Not a general JSON
+ * parser -- just enough for this project's small, flat config response,
+ * to avoid pulling in a JSON library for one scalar field. */
+static bool json_extract_uint(const char *json, const char *key, uint32_t *out)
 {
+    char needle[48];
+    snprintf(needle, sizeof(needle), "\"%s\"", key);
+    const char *pos = strstr(json, needle);
+    if (pos == NULL) {
+        return false;
+    }
+    pos = strchr(pos, ':');
+    if (pos == NULL) {
+        return false;
+    }
+    pos++;
+    while (*pos == ' ') {
+        pos++;
+    }
+    char *end;
+    unsigned long value = strtoul(pos, &end, 10);
+    if (end == pos) {
+        return false;
+    }
+    *out = (uint32_t)value;
+    return true;
+}
+
+/* GETs the server's /frame/config -- doubles as both the reachability
+ * check (any completed HTTP response means the socket-level connection
+ * succeeded) and the source of the server-configurable refresh interval. */
+static frame_server_config_t fetch_frame_config(const char *toolsserver)
+{
+    frame_server_config_t result = {
+        .reachable = false,
+        .refresh_interval_s = CONFIG_FRAME_SLEEP_INTERVAL_S,
+    };
+
     char url[160];
-    snprintf(url, sizeof(url), "http://%s/health", toolsserver);
+    snprintf(url, sizeof(url), "http://%s/frame/config", toolsserver);
 
     esp_http_client_config_t config = {
         .url = url,
@@ -124,13 +164,38 @@ static bool check_server_reachable(const char *toolsserver)
         .timeout_ms = CONFIG_FRAME_SERVER_CHECK_TIMEOUT_MS,
     };
     esp_http_client_handle_t client = esp_http_client_init(&config);
-    esp_err_t err = esp_http_client_perform(client);
-    esp_http_client_cleanup(client);
 
+    esp_err_t err = esp_http_client_open(client, 0);
     if (err != ESP_OK) {
         ESP_LOGW(TAG, "Server '%s' not reachable: %s", toolsserver, esp_err_to_name(err));
+        esp_http_client_cleanup(client);
+        return result;
     }
-    return err == ESP_OK;
+
+    esp_http_client_fetch_headers(client);
+    result.reachable = true;
+
+    char body[256];
+    int total = 0;
+    int n;
+    while (total < (int)sizeof(body) - 1 &&
+           (n = esp_http_client_read(client, body + total, sizeof(body) - 1 - total)) > 0) {
+        total += n;
+    }
+    body[total] = '\0';
+
+    esp_http_client_close(client);
+    esp_http_client_cleanup(client);
+
+    uint32_t interval;
+    if (json_extract_uint(body, "refresh_interval_s", &interval)) {
+        result.refresh_interval_s = interval;
+    } else {
+        ESP_LOGW(TAG, "'%s' response missing refresh_interval_s, using fallback %ds", url,
+                 (int)result.refresh_interval_s);
+    }
+
+    return result;
 }
 
 typedef struct {
@@ -212,10 +277,10 @@ void frame_client_run(const frame_config_t *cfg)
         }
     }
 
-    uint32_t sleep_seconds = CONFIG_FRAME_SLEEP_INTERVAL_S;
-    bool server_reachable = check_server_reachable(cfg->toolsserver);
+    frame_server_config_t server_cfg = fetch_frame_config(cfg->toolsserver);
+    uint32_t sleep_seconds = server_cfg.refresh_interval_s;
 
-    if (!server_reachable) {
+    if (!server_cfg.reachable) {
         /* Nothing's been drawn yet this cycle (aside from the optional
          * PENDING screen above), so this is cheap diagnostics without
          * compounding flashing. */