Turbocharge WiFi uploads with WebSocket + watchdog stability (#364)

## Summary

* **What is the goal of this PR?** Fix WiFi file transfer stability
issues (especially crashes during uploads) and improve upload speed via
WebSocket binary protocol. File transfers now don't really crash as
much, if they do it recovers and speed has gone form 50KB/s to 300+KB/s.

* **What changes are included?**
- **WebSocket upload support** - Adds WebSocket binary protocol for file
uploads, achieving faster speeds 335 KB/s vs HTTP multipart)
- **Watchdog stability fixes** - Adds `esp_task_wdt_reset()` calls
throughout upload path to prevent watchdog timeouts during:
    - File creation (FAT allocation can be slow)
    - SD card write operations
    - HTTP header parsing
    - WebSocket chunk processing
- **4KB write buffering** - Batches SD card writes to reduce I/O
overhead
- **WiFi health monitoring** - Detects WiFi disconnection in STA mode
and exits gracefully
- **Improved handleClient loop** - 500 iterations with periodic watchdog
resets and button checks for responsiveness
- **Progress bar improvements** - Fixed jumping/inaccurate progress by
capping local progress at 95% until server confirms completion
- **Exit button responsiveness** - Button now checked inside the
handleClient loop every 64 iterations
- **Reduced exit delays** - Decreased shutdown delays from ~850ms to
~140ms

**Files changed:**
- `platformio.ini` - Added WebSockets library dependency
- `CrossPointWebServer.cpp/h` - WebSocket server, upload buffering,
watchdog resets
- `CrossPointWebServerActivity.cpp` - WiFi monitoring, improved loop,
button handling
- `FilesPage.html` - WebSocket upload JavaScript with HTTP fallback

## Additional Context

- WebSocket uses 4KB chunks with backpressure management to prevent
ESP32 buffer overflow
- Falls back to HTTP automatically if WebSocket connection fails
- The main bottleneck now is SD card write speed (~44% of transfer
time), not WiFi
- STA mode was more prone to crashes than AP mode due to external
network factors; WiFi health monitoring helps detect and handle
disconnections gracefully

---

### AI Usage

Did you use AI tools to help write this code? _**YES**_ Claude did it
ALL, I have no idea what I am doing, but my books transfer fast now.

---------

Co-authored-by: Claude <noreply@anthropic.com>

src/activities/network/CrossPointWebServerActivity.cpp

@@ -4,6 +4,7 @@
 #include <ESPmDNS.h>
 #include <GfxRenderer.h>
 #include <WiFi.h>
+#include <esp_task_wdt.h>
 #include <qrcode.h>
 
 #include <cstddef>
@@ -83,9 +84,8 @@ void CrossPointWebServerActivity::onExit() {
     dnsServer = nullptr;
   }
 
-  // CRITICAL: Wait for LWIP stack to flush any pending packets
-  Serial.printf("[%lu] [WEBACT] Waiting 500ms for network stack to flush pending packets...\n", millis());
-  delay(500);
+  // Brief wait for LWIP stack to flush pending packets
+  delay(50);
 
   // Disconnect WiFi gracefully
   if (isApMode) {
@@ -95,11 +95,11 @@ void CrossPointWebServerActivity::onExit() {
     Serial.printf("[%lu] [WEBACT] Disconnecting WiFi (graceful)...\n", millis());
     WiFi.disconnect(false);  // false = don't erase credentials, send disconnect frame
   }
-  delay(100);  // Allow disconnect frame to be sent
+  delay(30);  // Allow disconnect frame to be sent
 
   Serial.printf("[%lu] [WEBACT] Setting WiFi mode OFF...\n", millis());
   WiFi.mode(WIFI_OFF);
-  delay(100);  // Allow WiFi hardware to fully power down
+  delay(30);  // Allow WiFi hardware to power down
 
   Serial.printf("[%lu] [WEBACT] [MEM] Free heap after WiFi disconnect: %d bytes\n", millis(), ESP.getFreeHeap());
 
@@ -283,8 +283,28 @@ void CrossPointWebServerActivity::loop() {
       dnsServer->processNextRequest();
     }
 
-    // Handle web server requests - call handleClient multiple times per loop
-    // to improve responsiveness and upload throughput
+    // STA mode: Monitor WiFi connection health
+    if (!isApMode && webServer && webServer->isRunning()) {
+      static unsigned long lastWifiCheck = 0;
+      if (millis() - lastWifiCheck > 2000) {  // Check every 2 seconds
+        lastWifiCheck = millis();
+        const wl_status_t wifiStatus = WiFi.status();
+        if (wifiStatus != WL_CONNECTED) {
+          Serial.printf("[%lu] [WEBACT] WiFi disconnected! Status: %d\n", millis(), wifiStatus);
+          // Show error and exit gracefully
+          state = WebServerActivityState::SHUTTING_DOWN;
+          updateRequired = true;
+          return;
+        }
+        // Log weak signal warnings
+        const int rssi = WiFi.RSSI();
+        if (rssi < -75) {
+          Serial.printf("[%lu] [WEBACT] Warning: Weak WiFi signal: %d dBm\n", millis(), rssi);
+        }
+      }
+    }
+
+    // Handle web server requests - maximize throughput with watchdog safety
     if (webServer && webServer->isRunning()) {
       const unsigned long timeSinceLastHandleClient = millis() - lastHandleClientTime;
 
@@ -294,17 +314,32 @@ void CrossPointWebServerActivity::loop() {
                       timeSinceLastHandleClient);
       }
 
-      // Call handleClient multiple times to process pending requests faster
-      // This is critical for upload performance - HTTP file uploads send data
-      // in chunks and each handleClient() call processes incoming data
-      constexpr int HANDLE_CLIENT_ITERATIONS = 10;
-      for (int i = 0; i < HANDLE_CLIENT_ITERATIONS && webServer->isRunning(); i++) {
+      // Reset watchdog BEFORE processing - HTTP header parsing can be slow
+      esp_task_wdt_reset();
+
+      // Process HTTP requests in tight loop for maximum throughput
+      // More iterations = more data processed per main loop cycle
+      constexpr int MAX_ITERATIONS = 500;
+      for (int i = 0; i < MAX_ITERATIONS && webServer->isRunning(); i++) {
         webServer->handleClient();
+        // Reset watchdog every 32 iterations
+        if ((i & 0x1F) == 0x1F) {
+          esp_task_wdt_reset();
+        }
+        // Yield and check for exit button every 64 iterations
+        if ((i & 0x3F) == 0x3F) {
+          yield();
+          // Check for exit button inside loop for responsiveness
+          if (mappedInput.wasPressed(MappedInputManager::Button::Back)) {
+            onGoBack();
+            return;
+          }
+        }
       }
       lastHandleClientTime = millis();
     }
 
-    // Handle exit on Back button
+    // Handle exit on Back button (also check outside loop)
     if (mappedInput.wasPressed(MappedInputManager::Button::Back)) {
       onGoBack();
       return;