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/models/project.rs

@@ -45,6 +45,8 @@ pub struct Project {
     pub port_mappings: Vec<PortMapping>,
     #[serde(default)]
     pub claude_instructions: Option<String>,
+    #[serde(default)]
+    pub enabled_mcp_servers: Vec<String>,
     pub created_at: String,
     pub updated_at: String,
 }
@@ -130,6 +132,7 @@ impl Project {
             custom_env_vars: Vec::new(),
             port_mappings: Vec::new(),
             claude_instructions: None,
+            enabled_mcp_servers: Vec::new(),
             created_at: now.clone(),
             updated_at: now,
         }