Finish captive portal: NVS config, POST handler, STA connect, AP identity

Splits provisioning (softAP + captive portal + NVS-backed config) into
wifi_provisioning.c and home-network connection into frame_client.c.
/save_config now parses the form body and persists it to NVS; on boot the
device goes straight to STA mode if a config exists, retrying a few times
before falling back to provisioning if the home network is unreachable.

The provisioning AP is now always named ESPRESSO with a random per-device
password (generated once, persisted in NVS) instead of a fixed Kconfig
value, drawn from a charset that avoids visually ambiguous characters since
it'll be read off the e-ink panel and possibly typed by hand.

firmware/main/Kconfig.projbuild

@@ -1,16 +1,14 @@
-menu "Example Configuration"
+menu "ESPresso Frame Configuration"
 
-    config ESP_WIFI_SSID
-        string "SoftAP SSID"
-        default "esp32_ssid"
+    config ESP_AP_SSID
+        string "Provisioning softAP SSID"
+        default "ESPRESSO"
         help
-            SSID (network name) to set up the softAP with.
-
-    config ESP_WIFI_PASSWORD
-        string "SoftAP Password"
-        default "esp32_pwd"
-        help
-            WiFi password (WPA or WPA2) for the example to use for the softAP.
+            SSID of the softAP the device brings up when it needs provisioning.
+            The password is generated randomly per device on first boot and
+            shown on the e-ink panel both as a WiFi-join QR code and as
+            plaintext underneath it, so it can be typed in by hand on a
+            desktop/laptop that can't scan the QR.
 
     config ESP_MAX_STA_CONN
         int "Maximal STA connections"
@@ -29,4 +27,19 @@ menu "Example Configuration"
         default y
         help
             Enables more modern DHCP-based Option 114 to provide clients with the captive portal URI
+
+    config FRAME_STA_CONNECT_MAX_RETRIES
+        int "Home WiFi connect max retries before falling back to provisioning"
+        default 3
+        help
+            Number of failed connection attempts to the stored home WiFi network
+            before the device gives up and re-enters provisioning (softAP) mode.
+
+    config FRAME_STA_CONNECT_TIMEOUT_MS
+        int "Home WiFi connect timeout per attempt (ms)"
+        default 15000
+        help
+            How long to wait for an IP address on each connection attempt to the
+            stored home WiFi network before counting it as a failed retry.
+
 endmenu