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/Source/service/displayidle/McpScreensaver.h

@@ -0,0 +1,30 @@
+#pragma once
+#ifdef ESP_PLATFORM
+
+#include "Screensaver.h"
+#include <cstdint>
+
+namespace tt::service::displayidle {
+
+/**
+ * MCP Screen screensaver.
+ * Creates a full-screen LVGL canvas on the overlay and registers it in
+ * McpSystemState so that MCP HTTP draw commands can paint to it.
+ * Dismissed by a touch event (handled by the parent DisplayIdle overlay).
+ */
+class McpScreensaver final : public Screensaver {
+    uint16_t* framebuffer = nullptr;
+    size_t framebufferSize = 0;
+
+public:
+    McpScreensaver() = default;
+    ~McpScreensaver() override = default;
+
+    void start(lv_obj_t* overlay, lv_coord_t screenW, lv_coord_t screenH) override;
+    void stop() override;
+    void update(lv_coord_t screenW, lv_coord_t screenH) override;
+};
+
+} // namespace tt::service::displayidle
+
+#endif // ESP_PLATFORM