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/activities/home/HomeActivity.cpp

@@ -6,7 +6,7 @@
 #include "config.h"
 
 namespace {
-constexpr int menuItemCount = 2;
+constexpr int menuItemCount = 3;
 }
 
 void HomeActivity::taskTrampoline(void* param) {
@@ -51,6 +51,8 @@ void HomeActivity::loop() {
     if (selectorIndex == 0) {
       onReaderOpen();
     } else if (selectorIndex == 1) {
+      onFileTransferOpen();
+    } else if (selectorIndex == 2) {
       onSettingsOpen();
     }
   } else if (prevPressed) {
@@ -84,7 +86,8 @@ void HomeActivity::render() const {
   // Draw selection
   renderer.fillRect(0, 60 + selectorIndex * 30 + 2, pageWidth - 1, 30);
   renderer.drawText(UI_FONT_ID, 20, 60, "Read", selectorIndex != 0);
-  renderer.drawText(UI_FONT_ID, 20, 90, "Settings", selectorIndex != 1);
+  renderer.drawText(UI_FONT_ID, 20, 90, "File transfer", selectorIndex != 1);
+  renderer.drawText(UI_FONT_ID, 20, 120, "Settings", selectorIndex != 2);
 
   renderer.drawRect(25, pageHeight - 40, 106, 40);
   renderer.drawText(UI_FONT_ID, 25 + (105 - renderer.getTextWidth(UI_FONT_ID, "Back")) / 2, pageHeight - 35, "Back");

src/activities/home/HomeActivity.h

@@ -14,6 +14,7 @@ class HomeActivity final : public Activity {
   bool updateRequired = false;
   const std::function<void()> onReaderOpen;
   const std::function<void()> onSettingsOpen;
+  const std::function<void()> onFileTransferOpen;
 
   static void taskTrampoline(void* param);
   [[noreturn]] void displayTaskLoop();
@@ -21,8 +22,11 @@ class HomeActivity final : public Activity {
 
  public:
   explicit HomeActivity(GfxRenderer& renderer, InputManager& inputManager, const std::function<void()>& onReaderOpen,
-                        const std::function<void()>& onSettingsOpen)
-      : Activity(renderer, inputManager), onReaderOpen(onReaderOpen), onSettingsOpen(onSettingsOpen) {}
+                        const std::function<void()>& onSettingsOpen, const std::function<void()>& onFileTransferOpen)
+      : Activity(renderer, inputManager),
+        onReaderOpen(onReaderOpen),
+        onSettingsOpen(onSettingsOpen),
+        onFileTransferOpen(onFileTransferOpen) {}
   void onEnter() override;
   void onExit() override;
   void loop() override;