feat: add admin password change via /settings page

Backend:
- DB: add UpdateAdminPasswordHash(id, hash) in queries_users.go
- Auth: add ChangePassword(username, old, new) method that verifies
  old password, validates length >= 8, and stores bcrypt hash
- Handlers: add handleChangePassword on PUT /api/auth/password (auth required)
- Router: register route inside protected group

Frontend:
- api.ts: add changePassword(oldPassword, newPassword)
- Settings.tsx: new page with form (current + new + confirm), client-side
  validation, success/error feedback
- App.tsx: add /settings route
- Layout.tsx: add 'Ajustes' nav item

web/src/App.tsx

@@ -9,6 +9,7 @@ import Users from "./pages/Users";
 import Samba from "./pages/Samba";
 import Nfs from "./pages/Nfs";
 import Log from "./pages/Log";
+import Settings from "./pages/Settings";
 
 type AuthState = { loading: boolean; authenticated: boolean; username: string };
 
@@ -54,6 +55,7 @@ export default function App() {
           <Route path="/samba" element={<Samba />} />
           <Route path="/nfs" element={<Nfs />} />
           <Route path="/log" element={<Log />} />
+          <Route path="/settings" element={<Settings />} />
           <Route path="*" element={<Navigate to="/" replace />} />
         </Route>
       </Routes>

web/src/api.ts

@@ -87,6 +87,8 @@ export const api = {
   login: (username: string, password: string) =>
     request<{ username: string }>("POST", "/auth/login", { username, password }),
   logout: () => request<{ ok: boolean }>("POST", "/auth/logout"),
+  changePassword: (oldPassword: string, newPassword: string) =>
+    request<{ ok: boolean }>("PUT", "/auth/password", { old_password: oldPassword, new_password: newPassword }),
 
   // dirty / apply
   dirty: () => request<{ modules: DirtyModule[] | null }>("GET", "/dirty"),

web/src/components/Layout.tsx

@@ -8,6 +8,7 @@ const navItems = [
   { to: "/samba", label: "SMB / Samba" },
   { to: "/nfs", label: "NFS" },
   { to: "/log", label: "Historial" },
+  { to: "/settings", label: "Ajustes" },
 ];
 
 export default function Layout({ username, onLogout }: { username: string; onLogout: () => void }) {

web/src/pages/Settings.tsx

@@ -0,0 +1,97 @@
+import { FormEvent, useState } from "react";
+import { api } from "../api";
+
+export default function Settings() {
+  const [oldPassword, setOldPassword] = useState("");
+  const [newPassword, setNewPassword] = useState("");
+  const [confirm, setConfirm] = useState("");
+  const [error, setError] = useState<string | null>(null);
+  const [success, setSuccess] = useState(false);
+  const [loading, setLoading] = useState(false);
+
+  async function handleSubmit(e: FormEvent) {
+    e.preventDefault();
+    setError(null);
+    setSuccess(false);
+
+    if (newPassword.length < 8) {
+      setError("La nueva contraseña debe tener al menos 8 caracteres.");
+      return;
+    }
+    if (newPassword !== confirm) {
+      setError("La nueva contraseña y la confirmación no coinciden.");
+      return;
+    }
+
+    setLoading(true);
+    try {
+      await api.changePassword(oldPassword, newPassword);
+      setSuccess(true);
+      setOldPassword("");
+      setNewPassword("");
+      setConfirm("");
+    } catch (err) {
+      setError(err instanceof Error ? err.message : "Error al cambiar la contraseña.");
+    } finally {
+      setLoading(false);
+    }
+  }
+
+  return (
+    <div className="space-y-6">
+      <h1 className="text-2xl font-bold text-white">Ajustes</h1>
+
+      <div className="card max-w-lg">
+        <h2 className="mb-4 text-lg font-semibold text-white">Cambiar contraseña</h2>
+
+        {error && (
+          <div className="mb-4 rounded-md bg-red-500/15 px-3 py-2 text-sm text-red-200">{error}</div>
+        )}
+        {success && (
+          <div className="mb-4 rounded-md bg-emerald-500/15 px-3 py-2 text-sm text-emerald-200">
+            Contraseña cambiada correctamente.
+          </div>
+        )}
+
+        <form onSubmit={handleSubmit} className="space-y-4">
+          <div>
+            <label className="label">Contraseña actual</label>
+            <input
+              className="input"
+              type="password"
+              value={oldPassword}
+              onChange={(e) => setOldPassword(e.target.value)}
+              autoComplete="current-password"
+            />
+          </div>
+          <div>
+            <label className="label">Nueva contraseña</label>
+            <input
+              className="input"
+              type="password"
+              value={newPassword}
+              onChange={(e) => setNewPassword(e.target.value)}
+              autoComplete="new-password"
+            />
+            <p className="mt-1 text-xs text-slate-500">Mínimo 8 caracteres.</p>
+          </div>
+          <div>
+            <label className="label">Confirmar nueva contraseña</label>
+            <input
+              className="input"
+              type="password"
+              value={confirm}
+              onChange={(e) => setConfirm(e.target.value)}
+              autoComplete="new-password"
+            />
+          </div>
+          <div className="pt-2">
+            <button className="btn-primary" type="submit" disabled={loading}>
+              {loading ? "Guardando..." : "Guardar contraseña"}
+            </button>
+          </div>
+        </form>
+      </div>
+    </div>
+  );
+}