Add AmneziaWG 3.1 support with key reissue

Protocol generation is now derived from awg1.conf rather than stored: an
interface speaks 3.1 when HeaderProtectionKey is set and 2.0 otherwise. The
3.0/3.1-only keys (HeaderProtectionKey, ContentPaddingAddition, the
Rekey/Reject/Keepalive timers, MaxHandshakeAttempts, RandomTrailers,
DisableCookies) are read from the conf and emitted only when non-empty, so 2.0
output stays byte-identical — verified by diffing both the client .conf and the
full vpn:// string against the previous implementation.

Changing generation invalidates every issued vpn:// key, so add
POST /api/users/reissue. It recovers the client private key from inside the
stored blob (the only place it exists), keeping ip/pub_key/psk_key intact, so
nothing changes on the wire and users only need to re-import. users gains
key_gen and vpn_key_prev, and users.db is snapshotted before the pass.

install.sh gates 3.1 on module 3.x and kernel >= 5.5 (header protection needs
the chacha library API, absent before 5.5) and falls back to writing a 2.0 conf
instead of aborting, so older kernels keep working as before.
HeaderProtectionKey is generated with awg genpsk and preserved on KEEP_DATA.

The panel marks keys issued on a different generation and offers to reissue
them; /api/v1 now returns gen alongside name/ip/vpn_key.

Also add MIT license headers across the awg-ui sources.

awg-ui/src/App.tsx

@@ -36,7 +36,7 @@ export default function App() {
     // вкладок (юзеры, ключи) грузят сами вкладки.
     const startSession = useCallback(async (tok: string) => {
         const { data: h } = await axios.get('/health', { headers: { Authorization: `Bearer ${tok}` } });
-        setServerInfo({ name: h.server || 'VPN', ip: h.ip || '', peers: h.awg?.peers ?? 0 });
+        setServerInfo({ name: h.server || 'VPN', ip: h.ip || '', peers: h.awg?.peers ?? 0, gen: h.gen || '2' });
         setStatusText('online:' + (h.server || 'ok') + ' · peers: ' + (h.awg?.peers ?? '?'));
     }, []);