Add connect to Wifi and File Manager Webserver (#41)

## Summary

- **What is the goal of this PR?**  
Implements wireless EPUB file management via a built-in web server,
enabling users to upload, browse, organize, and delete EPUB files from
any device on the same WiFi network without needing a computer cable
connection.

- **What changes are included?**
- **New Web Server**
([`CrossPointWebServer.cpp`](src/CrossPointWebServer.cpp),
[`CrossPointWebServer.h`](src/CrossPointWebServer.h)):
    - HTTP server on port 80 with a responsive HTML/CSS interface
    - Home page showing device status (version, IP, free memory)
    - File Manager with folder navigation and breadcrumb support
    - EPUB file upload with progress tracking
    - Folder creation and file/folder deletion
    - XSS protection via HTML escaping
- Hidden system folders (`.` prefixed, "System Volume Information",
"XTCache")
  
- **WiFi Screen** ([`WifiScreen.cpp`](src/screens/WifiScreen.cpp),
[`WifiScreen.h`](src/screens/WifiScreen.h)):
    - Network scanning with signal strength indicators
    - Visual indicators for encrypted (`*`) and saved (`+`) networks
- State machine managing: scanning, network selection, password entry,
connecting, save/forget prompts
    - 15-second connection timeout handling
    - Integration with web server (starts on connect, stops on exit)
  
- **WiFi Credential Storage**
([`WifiCredentialStore.cpp`](src/WifiCredentialStore.cpp),
[`WifiCredentialStore.h`](src/WifiCredentialStore.h)):
    - Persistent storage in `/sd/.crosspoint/wifi.bin`
- XOR obfuscation for stored passwords (basic protection against casual
reading)
    - Up to 8 saved networks with add/remove/update operations
  
- **On-Screen Keyboard**
([`OnScreenKeyboard.cpp`](src/screens/OnScreenKeyboard.cpp),
[`OnScreenKeyboard.h`](src/screens/OnScreenKeyboard.h)):
    - Reusable QWERTY keyboard component with shift support
    - Special keys: Shift, Space, Backspace, Done
    - Support for password masking mode
  
- **Settings Screen Integration**
([`SettingsScreen.h`](src/screens/SettingsScreen.h)):
    - Added WiFi action to navigate to the new WiFi screen
  
  - **Documentation** ([`docs/webserver.md`](docs/webserver.md)):
- Comprehensive user guide covering WiFi setup, web interface usage,
file management, troubleshooting, and security notes
    - See this for more screenshots!
- Working "displays the right way in GitHub" on my repo:
https://github.com/olearycrew/crosspoint-reader/blob/feature/connect-to-wifi/docs/webserver.md

**Video demo**


https://github.com/user-attachments/assets/283e32dc-2d9f-4ae2-848e-01f41166a731

## Additional Context

- **Security considerations**: The web server has no
authentication—anyone on the same WiFi network can access files. This is
documented as a limitation, recommending use only on trusted private
networks. Password obfuscation in the credential store is XOR-based, not
cryptographically secure.

- **Memory implications**: The web server and WiFi stack consume
significant memory. The implementation properly cleans up (stops server,
disconnects WiFi, sets `WIFI_OFF` mode) when exiting the WiFi screen to
free resources.

- **Async operations**: Network scanning and connection use async
patterns with FreeRTOS tasks to prevent blocking the UI. The display
task handles rendering on a dedicated thread with mutex protection.

- **Browser compatibility**: The web interface uses standard
HTML5/CSS3/JavaScript and is tested to work with all modern browsers on
desktop and mobile.

---------

Co-authored-by: Dave Allie <dave@daveallie.com>

src/main.cpp

@@ -5,6 +5,7 @@
 #include <InputManager.h>
 #include <SD.h>
 #include <SPI.h>
+#include <WiFi.h>
 #include <builtinFonts/bookerly_2b.h>
 #include <builtinFonts/bookerly_bold_2b.h>
 #include <builtinFonts/bookerly_bold_italic_2b.h>
@@ -19,6 +20,7 @@
 #include "activities/boot_sleep/BootActivity.h"
 #include "activities/boot_sleep/SleepActivity.h"
 #include "activities/home/HomeActivity.h"
+#include "activities/network/CrossPointWebServerActivity.h"
 #include "activities/reader/ReaderActivity.h"
 #include "activities/settings/SettingsActivity.h"
 #include "activities/util/FullScreenMessageActivity.h"
@@ -64,6 +66,7 @@ void exitActivity() {
   if (currentActivity) {
     currentActivity->onExit();
     delete currentActivity;
+    currentActivity = nullptr;
   }
 }
 
@@ -136,6 +139,11 @@ void onGoToReader(const std::string& initialEpubPath) {
 }
 void onGoToReaderHome() { onGoToReader(std::string()); }
 
+void onGoToFileTransfer() {
+  exitActivity();
+  enterNewActivity(new CrossPointWebServerActivity(renderer, inputManager, onGoHome));
+}
+
 void onGoToSettings() {
   exitActivity();
   enterNewActivity(new SettingsActivity(renderer, inputManager, onGoHome));
@@ -143,7 +151,7 @@ void onGoToSettings() {
 
 void onGoHome() {
   exitActivity();
-  enterNewActivity(new HomeActivity(renderer, inputManager, onGoToReaderHome, onGoToSettings));
+  enterNewActivity(new HomeActivity(renderer, inputManager, onGoToReaderHome, onGoToSettings, onGoToFileTransfer));
 }
 
 void setup() {
@@ -195,7 +203,10 @@ void setup() {
 }
 
 void loop() {
-  delay(10);
+  static unsigned long lastLoopTime = 0;
+  static unsigned long maxLoopDuration = 0;
+
+  unsigned long loopStartTime = millis();
 
   static unsigned long lastMemPrint = 0;
   if (Serial && millis() - lastMemPrint >= 10000) {
@@ -226,7 +237,29 @@ void loop() {
     return;
   }
 
+  unsigned long activityStartTime = millis();
   if (currentActivity) {
     currentActivity->loop();
   }
+  unsigned long activityDuration = millis() - activityStartTime;
+
+  unsigned long loopDuration = millis() - loopStartTime;
+  if (loopDuration > maxLoopDuration) {
+    maxLoopDuration = loopDuration;
+    if (maxLoopDuration > 50) {
+      Serial.printf("[%lu] [LOOP] New max loop duration: %lu ms (activity: %lu ms)\n", millis(), maxLoopDuration,
+                    activityDuration);
+    }
+  }
+
+  lastLoopTime = loopStartTime;
+
+  // Add delay at the end of the loop to prevent tight spinning
+  // When an activity requests skip loop delay (e.g., webserver running), use yield() for faster response
+  // Otherwise, use longer delay to save power
+  if (currentActivity && currentActivity->skipLoopDelay()) {
+    yield();  // Give FreeRTOS a chance to run tasks, but return immediately
+  } else {
+    delay(10);  // Normal delay when no activity requires fast response
+  }
 }