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/components/layout/Sidebar.tsx

@@ -1,6 +1,7 @@
 import { useShallow } from "zustand/react/shallow";
 import { useAppState } from "../../store/appState";
 import ProjectList from "../projects/ProjectList";
+import McpPanel from "../mcp/McpPanel";
 import SettingsPanel from "../settings/SettingsPanel";
 
 export default function Sidebar() {
@@ -8,35 +9,37 @@ export default function Sidebar() {
     useShallow(s => ({ sidebarView: s.sidebarView, setSidebarView: s.setSidebarView }))
   );
 
+  const tabCls = (view: typeof sidebarView) =>
+    `flex-1 px-3 py-2 text-sm font-medium transition-colors ${
+      sidebarView === view
+        ? "text-[var(--accent)] border-b-2 border-[var(--accent)]"
+        : "text-[var(--text-secondary)] hover:text-[var(--text-primary)]"
+    }`;
+
   return (
     <div className="flex flex-col h-full w-[25%] min-w-56 max-w-80 bg-[var(--bg-secondary)] border border-[var(--border-color)] rounded-lg overflow-hidden">
       {/* Nav tabs */}
       <div className="flex border-b border-[var(--border-color)]">
-        <button
-          onClick={() => setSidebarView("projects")}
-          className={`flex-1 px-3 py-2 text-sm font-medium transition-colors ${
-            sidebarView === "projects"
-              ? "text-[var(--accent)] border-b-2 border-[var(--accent)]"
-              : "text-[var(--text-secondary)] hover:text-[var(--text-primary)]"
-          }`}
-        >
+        <button onClick={() => setSidebarView("projects")} className={tabCls("projects")}>
           Projects
         </button>
-        <button
-          onClick={() => setSidebarView("settings")}
-          className={`flex-1 px-3 py-2 text-sm font-medium transition-colors ${
-            sidebarView === "settings"
-              ? "text-[var(--accent)] border-b-2 border-[var(--accent)]"
-              : "text-[var(--text-secondary)] hover:text-[var(--text-primary)]"
-          }`}
-        >
+        <button onClick={() => setSidebarView("mcp")} className={tabCls("mcp")}>
+          MCP
+        </button>
+        <button onClick={() => setSidebarView("settings")} className={tabCls("settings")}>
           Settings
         </button>
       </div>
 
       {/* Content */}
       <div className="flex-1 overflow-y-auto overflow-x-hidden p-1 min-w-0">
-        {sidebarView === "projects" ? <ProjectList /> : <SettingsPanel />}
+        {sidebarView === "projects" ? (
+          <ProjectList />
+        ) : sidebarView === "mcp" ? (
+          <McpPanel />
+        ) : (
+          <SettingsPanel />
+        )}
       </div>
     </div>
   );