feat: integrate MCP server and screen override into system firmware

- Add McpSettings (load/save mcpEnabled, mcpOverrideEnabled to /data/service/mcp/settings.properties)
- Add McpSystem: global state, canvas drawing (RGB565, BMP, PBM, text), I2S audio (WAV, MP3 via minimp3, tone, record)
- Add McpHandler: unauthenticated POST /api/mcp (JSON-RPC 2.0, 13 tools) and POST /api/screen/raw endpoints
- Route MCP endpoints before Basic Auth check in WebServerService::handleApiPost
- Start HTTP server at boot if either webServerEnabled OR mcpEnabled is set
- Fix setEnabled: start unconditionally when called with true; only stop if both WS and MCP are disabled
- Add McpSettingsApp (Settings category): toggle MCP service + screen override, show live endpoint URL
- Add McpOverrideApp (appId one.tactility.mcpscreen): full-screen LVGL canvas for LLM-driven display override
- Register both new apps in Tactility.cpp

Tactility/Include/Tactility/settings/McpSettings.h

@@ -0,0 +1,15 @@
+#pragma once
+
+namespace tt::settings::mcp {
+
+struct McpSettings {
+    bool mcpEnabled = false;            // Enable MCP server endpoints on system web server
+    bool mcpOverrideEnabled = true;     // Automatically switch to override screen on drawing commands
+};
+
+bool load(McpSettings& settings);
+McpSettings getDefault();
+McpSettings loadOrGetDefault();
+bool save(const McpSettings& settings);
+
+} // namespace tt::settings::mcp