fix(provisioning): stop redrawing the QR on every poll, add WiFi-fail retry screen

Two related fixes that together let the post-WiFi-setup window be quiet:

1. operation.h 204/404: skip the panel redraw entirely. The panel already
   holds the right thing — setup QR if no image has ever been painted
   (img_id == -1), or a real photo if img_id >= 0. Redrawing the QR every
   15s during the bootstrap claim window put the e-ink into a perpetual
   ~20s mid-refresh loop and risked ghosting. Tests updated to assert
   no redraw on either sub-case.

2. main.cpp WiFi-fail path: drop the epd_fill(RED) + 3s delay + AP
   re-redraw sequence (~43s of e-ink work that destroyed the QR mid-flow)
   and replace with a single repaint of a new "Connection Failed — try
   again" Step 1/2 screen with red accents. gen_screens.py grows a
   gen_ap_retry() variant that recolors yellow → red and swaps the
   header/QR labels; the result is shipped as ap_bg_retry.bin alongside
   ap_bg.bin in LittleFS. epd.h exposes epd_draw_ap_screen_retry().

src/epd.h

@@ -20,4 +20,7 @@ void epd_draw_qr(QRCode* qr, uint8_t cellPx, uint8_t bg, uint8_t fg);
 
 // Draw the setup screen: pre-rendered background from LittleFS with QR overlaid.
 void epd_draw_ap_screen(QRCode* qr);
+// Same layout as ap_screen but with red accents and a "Connection Failed —
+// try again" label, served after a failed WiFi join attempt.
+void epd_draw_ap_screen_retry(QRCode* qr);
 void epd_draw_setup_screen(QRCode* qr);

src/main.cpp

@@ -25,13 +25,17 @@ static String g_req_pass;
 
 // ── QR helpers ───────────────────────────────────────────────────────────────
 
-static void show_ap_qr(const String& apSsid) {
+static void show_ap_qr(const String& apSsid, bool retry = false) {
     String content = "WIFI:S:" + apSsid + ";T:nopass;;";
 
     QRCode qr;
     uint8_t buf[qrcode_getBufferSize(5)];
     qrcode_initText(&qr, buf, 5, ECC_LOW, content.c_str());
-    epd_draw_ap_screen(&qr);
+    if (retry) {
+        epd_draw_ap_screen_retry(&qr);
+    } else {
+        epd_draw_ap_screen(&qr);
+    }
 }
 
 static void show_setup_qr(const String& mac) {
@@ -116,7 +120,7 @@ static void handle_captive() {
 
 // ── WiFi provisioning ─────────────────────────────────────────────────────────
 
-static void enter_provisioning(const String& mac) {
+static void enter_provisioning(const String& mac, bool retry = false) {
     // Derive AP name from last 4 hex chars of MAC (no colons)
     String suffix = mac;
     suffix.replace(":", "");
@@ -124,7 +128,7 @@ static void enter_provisioning(const String& mac) {
     suffix.toUpperCase();
     String apSsid = "PictureFrame-" + suffix;
 
-    Serial.println("AP: " + apSsid);
+    Serial.println(retry ? "AP (retry): " + apSsid : "AP: " + apSsid);
 
     WiFi.disconnect(true);
     WiFi.mode(WIFI_AP);
@@ -143,8 +147,11 @@ static void enter_provisioning(const String& mac) {
     server.onNotFound(handle_root);
     server.begin();
 
+    // On retry, repaint with red accents + "Connection Failed — try again"
+    // label so the user has a clear visual signal that their last credential
+    // entry didn't work. On first entry, paint the standard yellow Step 1/2.
     epd_init();
-    show_ap_qr(apSsid);
+    show_ap_qr(apSsid, retry);
     epd_sleep();
 
     g_provisioning = true;
@@ -267,13 +274,11 @@ void loop() {
         // serves an image — no need for an artificial display delay here.
         normal_operation(mac);
     } else {
-        // Connection failed — fill red, restart AP
-        epd_init();
-        epd_fill(COLOR_RED);
-        epd_sleep();
-        delay(3000);
-
-        // Re-enter provisioning to retry
-        enter_provisioning(mac);
+        // Connection failed — go back into AP mode with the red retry screen
+        // so the user sees "Connection Failed — try again" and can rescan.
+        // No epd_fill(COLOR_RED) detour: that would obliterate the QR for
+        // ~20 s and force a second redraw to put it back. One repaint into
+        // the retry screen is faster and clearer.
+        enter_provisioning(mac, /*retry=*/true);
     }
 }

src/operation.h

@@ -263,16 +263,21 @@ void normal_operation_impl(const String& mac, HTTP& http, const String& url, Pre
                 Serial.println("[op] recovery aborted: /img.bin not in LittleFS");
             }
         }
-    } else if (code == 204) {
+    } else if (code == 204 || code == 404) {
+        // No image to serve. Don't touch the panel — whatever's already
+        // displayed is the right thing:
+        //   • currentImgId == -1 → the setup QR is up (painted by
+        //     enter_provisioning after WiFi save). The 15s bootstrap poll
+        //     hits this branch every cycle until the user claims via
+        //     /setup/{mac}; redrawing the QR each time would put the panel
+        //     in a perpetual ~20s e-ink redraw loop and risk ghosting.
+        //   • currentImgId >= 0 → a real photo is up (server hiccup, asset
+        //     missing, image deleted). Don't paint the setup QR over the
+        //     user's photo; leave the last-good image alone.
+        // displayInitialized stays false → epd_sleep() at the bottom is
+        // also skipped, since the display was already in sleep from the
+        // previous cycle.
         http.end();
-        displayInitialized = true;
-        epd_init();
-        show_setup_qr(mac);
-    } else if (code == 404) {
-        http.end();
-        displayInitialized = true;
-        epd_init();
-        show_setup_qr(mac);
     } else {
         // Sync failed (5xx, timeout, malformed). Per FR38, the last-good image
         // must persist; only the border indicates the error. epd_draw_image_with_border

src/panels/waveshare73/v1/epd_driver.cpp

@@ -179,6 +179,12 @@ void epd_draw_ap_screen(QRCode* qr) {
     draw_from_lfs("/ap_bg.bin", COLOR_YELLOW, qr, 563, 185, 5);
 }
 
+void epd_draw_ap_screen_retry(QRCode* qr) {
+    // Same QR coordinates — only the bg .bin differs (red accents,
+    // "Connection Failed — try again" label).
+    draw_from_lfs("/ap_bg_retry.bin", COLOR_RED, qr, 563, 185, 5);
+}
+
 void epd_draw_setup_screen(QRCode* qr) {
     // SETUP_QR_X=553, SETUP_QR_Y=175, SETUP_QR_CELL=5  (must match gen_screens.py)
     draw_from_lfs("/setup_bg.bin", COLOR_GREEN, qr, 553, 175, 5);