Add persistent navbar with SSH Keys link

Refactor App.tsx to use Layout + Outlet pattern with sticky navbar
containing: Dashboard | Machines | Sync Pairs | Jobs | SSH Keys | Settings.
Clean up redundant button row from Dashboard and inline SSH Keys links.

web/src/pages/Machines.tsx

@@ -1,5 +1,4 @@
 import { useEffect, useState } from 'react';
-import { Link } from 'react-router-dom';
 import { api, Machine, SSHKey } from '../api/client';
 
 export default function Machines() {
@@ -77,10 +76,7 @@ export default function Machines() {
   return (
     <div className="p-6">
       <div className="flex justify-between items-center mb-6">
-        <div className="flex items-center gap-4">
-          <h1 className="text-2xl font-bold">Machines</h1>
-          <Link to="/ssh-keys" className="text-sm text-blue-400 hover:text-blue-300">Manage SSH Keys</Link>
-        </div>
+        <h1 className="text-2xl font-bold">Machines</h1>
         <button onClick={() => setShowForm(true)} className="bg-blue-600 hover:bg-blue-700 text-white px-4 py-2 rounded">
           Add Machine
         </button>