feat: add MCP server support with global library and per-project toggles

Add Model Context Protocol (MCP) server configuration support. Users can
define MCP servers globally (new sidebar tab) and enable them per-project.
Enabled servers are injected into containers as MCP_SERVERS_JSON env var
and merged into ~/.claude.json by the entrypoint.

Backend: McpServer model, McpStore (JSON + atomic writes), 4 CRUD commands,
container injection with fingerprint-based recreation detection.
Frontend: MCP sidebar tab, McpPanel/McpServerCard components, useMcpServers
hook, per-project MCP checkboxes in ProjectCard config.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/src-tauri/src/commands/mcp_commands.rs

@@ -0,0 +1,38 @@
+use tauri::State;
+
+use crate::models::McpServer;
+use crate::AppState;
+
+#[tauri::command]
+pub async fn list_mcp_servers(state: State<'_, AppState>) -> Result<Vec<McpServer>, String> {
+    Ok(state.mcp_store.list())
+}
+
+#[tauri::command]
+pub async fn add_mcp_server(
+    name: String,
+    state: State<'_, AppState>,
+) -> Result<McpServer, String> {
+    let name = name.trim().to_string();
+    if name.is_empty() {
+        return Err("MCP server name cannot be empty.".to_string());
+    }
+    let server = McpServer::new(name);
+    state.mcp_store.add(server)
+}
+
+#[tauri::command]
+pub async fn update_mcp_server(
+    server: McpServer,
+    state: State<'_, AppState>,
+) -> Result<McpServer, String> {
+    state.mcp_store.update(server)
+}
+
+#[tauri::command]
+pub async fn remove_mcp_server(
+    server_id: String,
+    state: State<'_, AppState>,
+) -> Result<(), String> {
+    state.mcp_store.remove(&server_id)
+}