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/network/WifiSelectionActivity.h

@@ -0,0 +1,108 @@
+#pragma once
+#include <freertos/FreeRTOS.h>
+#include <freertos/semphr.h>
+#include <freertos/task.h>
+
+#include <cstdint>
+#include <functional>
+#include <memory>
+#include <string>
+#include <vector>
+
+#include "../Activity.h"
+#include "../util/KeyboardEntryActivity.h"
+
+// Structure to hold WiFi network information
+struct WifiNetworkInfo {
+  std::string ssid;
+  int32_t rssi;
+  bool isEncrypted;
+  bool hasSavedPassword;  // Whether we have saved credentials for this network
+};
+
+// WiFi selection states
+enum class WifiSelectionState {
+  SCANNING,           // Scanning for networks
+  NETWORK_LIST,       // Displaying available networks
+  PASSWORD_ENTRY,     // Entering password for selected network
+  CONNECTING,         // Attempting to connect
+  CONNECTED,          // Successfully connected
+  SAVE_PROMPT,        // Asking user if they want to save the password
+  CONNECTION_FAILED,  // Connection failed
+  FORGET_PROMPT       // Asking user if they want to forget the network
+};
+
+/**
+ * WifiSelectionActivity is responsible for scanning WiFi APs and connecting to them.
+ * It will:
+ * - Enter scanning mode on entry
+ * - List available WiFi networks
+ * - Allow selection and launch KeyboardEntryActivity for password if needed
+ * - Save the password if requested
+ * - Call onComplete callback when connected or cancelled
+ *
+ * The onComplete callback receives true if connected successfully, false if cancelled.
+ */
+class WifiSelectionActivity final : public Activity {
+  TaskHandle_t displayTaskHandle = nullptr;
+  SemaphoreHandle_t renderingMutex = nullptr;
+  bool updateRequired = false;
+  WifiSelectionState state = WifiSelectionState::SCANNING;
+  int selectedNetworkIndex = 0;
+  std::vector<WifiNetworkInfo> networks;
+  const std::function<void(bool connected)> onComplete;
+
+  // Selected network for connection
+  std::string selectedSSID;
+  bool selectedRequiresPassword = false;
+
+  // On-screen keyboard for password entry
+  std::unique_ptr<KeyboardEntryActivity> keyboard;
+
+  // Connection result
+  std::string connectedIP;
+  std::string connectionError;
+
+  // Password to potentially save (from keyboard or saved credentials)
+  std::string enteredPassword;
+
+  // Whether network was connected using a saved password (skip save prompt)
+  bool usedSavedPassword = false;
+
+  // Save/forget prompt selection (0 = Yes, 1 = No)
+  int savePromptSelection = 0;
+  int forgetPromptSelection = 0;
+
+  // Connection timeout
+  static constexpr unsigned long CONNECTION_TIMEOUT_MS = 15000;
+  unsigned long connectionStartTime = 0;
+
+  static void taskTrampoline(void* param);
+  [[noreturn]] void displayTaskLoop();
+  void render() const;
+  void renderNetworkList() const;
+  void renderPasswordEntry() const;
+  void renderConnecting() const;
+  void renderConnected() const;
+  void renderSavePrompt() const;
+  void renderConnectionFailed() const;
+  void renderForgetPrompt() const;
+
+  void startWifiScan();
+  void processWifiScanResults();
+  void selectNetwork(int index);
+  void attemptConnection();
+  void checkConnectionStatus();
+  std::string getSignalStrengthIndicator(int32_t rssi) const;
+
+ public:
+  explicit WifiSelectionActivity(GfxRenderer& renderer, InputManager& inputManager,
+                                 const std::function<void(bool connected)>& onComplete)
+      : Activity(renderer, inputManager), onComplete(onComplete) {}
+  void onEnter() override;
+  void onExit() override;
+  void loop() override;
+
+  // Get the IP address after successful connection
+  const std::string& getConnectedIP() const { return connectedIP; }
+};