feat(mcp): restore MCP service with display charging toggle

- Restore MCP system (McpSystem 1900+ LOC, McpHandler, McpScreensaver, McpSettings, McpOverride apps)
- Use audio-stream instead of direct I2S for MP3 playback (fixes fast playback, uses native resampler)
- Register McpSettings + McpOverride in Tactility.cpp so Settings shows MCP Screen
- DisplaySettings: add ScreensaverType::McpScreen and disableScreensaverWhenCharging flag
- Display app: add 'Disable on charging' toggle, handles McpScreen screensaver type
- DisplayIdle: skip screensaver while charging, support McpScreen screensaver, isDeviceCharging check via PowerDevice
- WebServer: coexistence with MCP (McpHandler)
- Avoid formatting churn from previous commits

Squashed from range eaa248b0..8970143f (6 commits) into single clean commit

Tactility/Include/Tactility/settings/DisplaySettings.h

@@ -18,6 +18,7 @@ enum class ScreensaverType {
     Mystify,
     MatrixRain,
     StackChan,
+    McpScreen,
     Count           // Sentinel for bounds checking - must be last
 };
 
@@ -28,6 +29,7 @@ struct DisplaySettings {
     bool backlightTimeoutEnabled;
     uint32_t backlightTimeoutMs; // 0 = Never
     ScreensaverType screensaverType = ScreensaverType::BouncingBalls;
+    bool disableScreensaverWhenCharging = false;
 };
 
 /** Compares default settings with the function parameter to return the difference */

Tactility/Include/Tactility/settings/McpSettings.h

@@ -0,0 +1,14 @@
+#pragma once
+
+namespace tt::settings::mcp {
+
+struct McpSettings {
+    bool mcpEnabled = false;            // Enable MCP server endpoints on system web server
+};
+
+bool load(McpSettings& settings);
+McpSettings getDefault();
+McpSettings loadOrGetDefault();
+bool save(const McpSettings& settings);
+
+} // namespace tt::settings::mcp