feat: Add admin authentication for Settings panel

- Add sftpgo_admins table to auth.sqlite (synced from PostgreSQL admins)
- Add PgAdmin struct + sync_admins() method in sync.rs
- Add fetch_admins() method in pg_client.rs
- Add AdminLoginRequest/Response + admin_login() + verify_admin_token() in auth.rs
- Add POST /api/v2/admin/login + GET /api/v2/admin/verify endpoints in server.rs
- Add AdminLoginModal UI with password input + localStorage token in page.html
- Test password: admin123 (bcrypt hash updated in PostgreSQL admins table)

Architecture:
- Independent admin auth system (matches SFTPGo design)
- Admin sessions stored in-memory (24h validity)
- bcrypt password verification (cost=10)
- localStorage token persistence for UI
- Settings panel requires admin authentication

Files changed:
- data/init_auth_db.sql: +20 lines
- src/sync.rs: +100 lines
- src/pg_client.rs: +50 lines
- src/auth.rs: +60 lines
- src/server.rs: +50 lines
- src/page.html: +70 lines
Total: ~290 lines added

Tested: Admin sync, login, verify, UI modal all working

src/page.html

@@ -84,6 +84,18 @@ body.mb-locked .mb-tree-node:hover .mb-folder-actions{display:none!important}
 .mb-config-cancel-btn{background:#451a03;border:1px solid #fbbf24;color:#fbbf24;padding:2px 8px;border-radius:4px;cursor:pointer;font-size:11px}
 .mb-password-toggle{background:none;border:none;color:#60a5fa;cursor:pointer;font-size:14px;padding:0 4px}
 .mb-password-toggle:hover{color:#3b82f6}
+#mb-admin-modal{display:none;position:fixed;top:50%;left:50%;transform:translate(-50%,-50%);
+background:#1e293b;border:1px solid #334155;padding:24px;border-radius:8px;z-index:10000;min-width:280px}
+#mb-admin-modal.active{display:block}
+.mb-admin-title{color:#60a5fa;font-size:16px;font-weight:600;margin-bottom:16px}
+.mb-admin-input{background:#0f172a;border:1px solid #60a5fa;border-radius:4px;
+color:#e2e8f0;padding:8px 12px;width:100%;margin-bottom:12px;font-size:13px}
+.mb-admin-btn{background:#064e3b;border:1px solid #4ade80;color:#4ade80;
+padding:8px 16px;border-radius:4px;cursor:pointer;width:100%;font-size:13px}
+.mb-admin-btn:hover{background:#4ade80;color:#064e3b}
+.mb-admin-error{color:#ef4444;font-size:12px;margin-top:8px}
+.mb-admin-close{position:absolute;top:12px;right:12px;background:none;border:none;
+color:#64748b;font-size:18px;cursor:pointer}
 </style>
 <script src="https://cdn.jsdelivr.net/npm/mermaid@11/dist/mermaid.min.js"></script>
 <script>mermaid.initialize({startOnLoad:false,theme:'dark',themeVariables:{primaryColor:'#60a5fa',primaryTextColor:'#e2e8f0',lineColor:'#94a3b8'}})</script>
@@ -124,9 +136,85 @@ body.mb-locked .mb-tree-node:hover .mb-folder-actions{display:none!important}
 var _sv=false;
 
 function toggleSettings(){
-    _sv=!_sv;
-    document.getElementById("mb-settings-panel").classList.toggle("active",_sv);
-    if(_sv)loadSettings();
+    var token=localStorage.getItem('admin_token');
+    
+    if(token){
+        // Verify token validity
+        fetch('/api/v2/admin/verify',{
+            headers:{'Authorization':'Bearer '+token}
+        })
+        .then(function(r){return r.json()})
+        .then(function(d){
+            if(d.ok){
+                // Token valid, open settings
+                _sv=!_sv;
+                document.getElementById("mb-settings-panel").classList.toggle("active",_sv);
+                if(_sv)loadSettings();
+            }else{
+                // Token invalid, remove and show login
+                localStorage.removeItem('admin_token');
+                showAdminLoginModal();
+            }
+        })
+        .catch(function(e){
+            localStorage.removeItem('admin_token');
+            showAdminLoginModal();
+        });
+    }else{
+        // No token, show login
+        showAdminLoginModal();
+    }
+}
+
+function showAdminLoginModal(){
+    var m=document.getElementById('mb-admin-modal');
+    
+    if(!m){
+        m=document.createElement('div');
+        m.id='mb-admin-modal';
+        m.innerHTML='<button class=mb-admin-close onclick=this.parentElement.classList.remove("active")>✕</button>'+
+        '<div class=mb-admin-title>Admin Authentication Required</div>'+
+        '<input class=mb-admin-input type=password id=admin-password placeholder="Enter admin password">'+
+        '<button class=mb-admin-btn onclick=submitAdminLogin()>Login</button>'+
+        '<div class=mb-admin-error id=admin-error></div>';
+        document.body.appendChild(m);
+    }
+    
+    document.getElementById('admin-password').value='';
+    document.getElementById('admin-error').textContent='';
+    m.classList.add('active');
+    document.getElementById('admin-password').focus();
+}
+
+function submitAdminLogin(){
+    var pwd=document.getElementById('admin-password').value;
+    
+    if(!pwd){
+        document.getElementById('admin-error').textContent='Password required';
+        return;
+    }
+    
+    fetch('/api/v2/admin/login',{
+        method:'POST',
+        headers:{'Content-Type':'application/json'},
+        body:JSON.stringify({username:'admin',password:pwd})
+    })
+    .then(function(r){return r.json()})
+    .then(function(d){
+        if(d.token){
+            localStorage.setItem('admin_token',d.token);
+            document.getElementById('mb-admin-modal').classList.remove('active');
+            toast('Admin authenticated ✓');
+            toggleSettings(); // Re-open settings
+        }else{
+            document.getElementById('admin-error').textContent=d.error||'Login failed';
+            document.getElementById('admin-password').value='';
+            document.getElementById('admin-password').focus();
+        }
+    })
+    .catch(function(e){
+        document.getElementById('admin-error').textContent='Connection error: '+e;
+    });
 }
 
 function loadSettings(){
@@ -204,7 +292,7 @@ function editSetting(key,currentVal){
     editBtn.outerHTML="<button class=mb-config-save-btn onclick=saveSetting(\""+key+"\",\""+safeId+"\",\""+isPassword+"\")>Save</button><button class=mb-config-cancel-btn onclick=cancelEdit(\""+key+"\",\""+currentVal+"\")>Cancel</button>";
 }
 
-function saveSetting(key,safeId){
+function saveSetting(key,safeId,isPassword){
     var input=document.getElementById("config-input-"+safeId);
     if(!input)return;
     
@@ -225,6 +313,22 @@ function cancelEdit(key,currentVal){
     loadSettings();
 }
 
+function togglePassword(safeId,actualVal){
+    var valEl=document.getElementById("config-value-"+safeId);
+    if(!valEl)return;
+    
+    var toggleBtn=valEl.parentElement.querySelector(".mb-password-toggle");
+    if(!toggleBtn)return;
+    
+    if(valEl.textContent==="••••••••"){
+        valEl.textContent=decodeURIComponent(actualVal);
+        toggleBtn.textContent="🙈";
+    }else{
+        valEl.textContent="••••••••";
+        toggleBtn.textContent="👁";
+    }
+}
+
 function validateSettings(){
     fetch("/api/v2/config/validate").then(function(r){return r.json()}).then(function(d){
         if(d.ok)toast("✓ Settings valid");