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/WifiCredentialStore.h

@@ -0,0 +1,56 @@
+#pragma once
+#include <string>
+#include <vector>
+
+struct WifiCredential {
+  std::string ssid;
+  std::string password;  // Stored obfuscated in file
+};
+
+/**
+ * Singleton class for storing WiFi credentials on the SD card.
+ * Credentials are stored in /sd/.crosspoint/wifi.bin with basic
+ * XOR obfuscation to prevent casual reading (not cryptographically secure).
+ */
+class WifiCredentialStore {
+ private:
+  static WifiCredentialStore instance;
+  std::vector<WifiCredential> credentials;
+
+  static constexpr size_t MAX_NETWORKS = 8;
+
+  // Private constructor for singleton
+  WifiCredentialStore() = default;
+
+  // XOR obfuscation (symmetric - same for encode/decode)
+  void obfuscate(std::string& data) const;
+
+ public:
+  // Delete copy constructor and assignment
+  WifiCredentialStore(const WifiCredentialStore&) = delete;
+  WifiCredentialStore& operator=(const WifiCredentialStore&) = delete;
+
+  // Get singleton instance
+  static WifiCredentialStore& getInstance() { return instance; }
+
+  // Save/load from SD card
+  bool saveToFile() const;
+  bool loadFromFile();
+
+  // Credential management
+  bool addCredential(const std::string& ssid, const std::string& password);
+  bool removeCredential(const std::string& ssid);
+  const WifiCredential* findCredential(const std::string& ssid) const;
+
+  // Get all stored credentials (for UI display)
+  const std::vector<WifiCredential>& getCredentials() const { return credentials; }
+
+  // Check if a network is saved
+  bool hasSavedCredential(const std::string& ssid) const;
+
+  // Clear all credentials
+  void clearAll();
+};
+
+// Helper macro to access credentials store
+#define WIFI_STORE WifiCredentialStore::getInstance()