feat(clients,groups): client groups + sub-links export + dedicated groups page

Persistent client groups
- New ClientGroup model + client_groups table that holds empty
  (placeholder) groups so a user can define a label before any client
  references it. ListGroups merges these with the distinct group_name
  values already stored on clients and reports {name, clientCount}.
- ClientRecord gains group_name column; the model.Client wire shape
  gains a matching `group` JSON field that survives the
  inbound.settings → SyncInbound round-trip.
- Rename/Delete on a group mutates client_groups (rename row / delete
  row) AND propagates to all matching clients in ClientRecord and in
  every owning inbound's settings JSON, all in one transaction.

Bulk operations
- AssignGroup(emails, group) updates clients.group_name + patches each
  affected inbound's settings JSON in one read-modify-write per inbound.
  Empty group clears the label. Auto-creates the client_groups row when
  the user assigns to a brand-new name.
- BulkResetTraffic(emails) loops the existing single-reset path so the
  caller can zero traffic across a whole selection or a whole group.
- EmailsByGroup(name) returns just the email list (used by the groups
  page to fan a single bulk action over every member).

Endpoints (all under /panel/api/clients)
- GET  /groups                         — summaries with counts
- GET  /groups/:name/emails            — emails in a group
- POST /groups/create                  — empty placeholder group
- POST /groups/rename                  — rename (table + clients + JSON)
- POST /groups/delete                  — drop label everywhere (clients survive)
- POST /bulkAssignGroup                — assign N selected clients
- POST /bulkResetTraffic               — reset traffic on a list

Clients page UX
- New Group column (Actions → Client → Group → Inbounds → …) with a
  click-to-filter chip.
- FilterDrawer gains a multi-select Group filter whose options come
  from the new ClientPageResponse.groups field (sourced from ListGroups
  so empty/placeholder groups are pickable too).
- Single-client and bulk-add forms gain a Group AutoComplete pre-loaded
  with all known group names.
- New toolbar buttons when selection > 0: "Group ({n})" opens
  BulkAssignGroupModal, "Sub links ({n})" opens SubLinksModal.

Sub-links export modal (new SubLinksModal.tsx)
- Table of selected clients with their subscription URL (and JSON URL
  when subJsonEnable is on), per-row copy, Copy all, and Download as
  sub-links-<timestamp>.txt. Warns when subscription is disabled or
  none of the selected clients have a subId.

Dedicated Groups page (new pages/groups/GroupsPage.tsx)
- /groups route + sidebar entry (TagsOutlined icon) + page title key.
- Card-based layout matching Clients/Inbounds/Nodes — summary card with
  Total/Grouped/Empty stats, main card with Add Group button + table.
- Per-row More dropdown (icon-first column on the left): Sub links,
  Adjust (days+traffic), Reset traffic, Rename, Delete clients in
  group, Delete group (keep clients). Empty groups disable the
  client-targeted actions.
- Reuses SubLinksModal and ClientBulkAdjustModal — emails for the
  group are fetched on demand from GET /groups/:name/emails.

Other polish
- /groups + groups-page selectors added to page-shell.css and
  page-cards.css so the new page inherits the same background, padding,
  card borders, hover shadow, and summary-card padding.
- .card-toolbar gains a small vertical padding so the larger toolbar
  buttons (now default size, matching Inbounds) don't crowd the top of
  the card-head on Clients and Groups pages.

frontend/src/hooks/useClients.ts

@@ -55,6 +55,7 @@ export interface ClientQueryParams {
   autoRenew?: 'on' | 'off' | '';
   hasTgId?: 'yes' | 'no' | '';
   hasComment?: 'yes' | 'no' | '';
+  group?: string;
 }
 
 const DEFAULT_QUERY: ClientQueryParams = { page: 1, pageSize: 25 };
@@ -79,6 +80,7 @@ function buildQS(p: ClientQueryParams): string {
   if (p.autoRenew) sp.set('autoRenew', p.autoRenew);
   if (p.hasTgId) sp.set('hasTgId', p.hasTgId);
   if (p.hasComment) sp.set('hasComment', p.hasComment);
+  if (p.group) sp.set('group', p.group);
   return sp.toString();
 }
 
@@ -130,6 +132,7 @@ export function useClients() {
         && (prev.autoRenew ?? '') === (next.autoRenew ?? '')
         && (prev.hasTgId ?? '') === (next.hasTgId ?? '')
         && (prev.hasComment ?? '') === (next.hasComment ?? '')
+        && (prev.group ?? '') === (next.group ?? '')
       ) return prev;
       return next;
     });
@@ -169,6 +172,7 @@ export function useClients() {
   const total = listQuery.data?.total ?? 0;
   const filtered = listQuery.data?.filtered ?? 0;
   const summary = listQuery.data?.summary ?? DEFAULT_SUMMARY;
+  const allGroups = listQuery.data?.groups ?? [];
   const fetched = listQuery.data !== undefined;
   const loading = listQuery.isFetching;
 
@@ -230,6 +234,12 @@ export function useClients() {
     onSuccess: (msg) => { if (msg?.success) invalidateAll(); },
   });
 
+  const bulkAssignGroupMut = useMutation({
+    mutationFn: (body: { emails: string[]; group: string }) =>
+      HttpUtil.post('/panel/api/clients/bulkAssignGroup', body, JSON_HEADERS),
+    onSuccess: (msg) => { if (msg?.success) invalidateAll(); },
+  });
+
   const updateMut = useMutation({
     mutationFn: ({ email, client }: { email: string; client: unknown }) =>
       HttpUtil.post(`/panel/api/clients/update/${encodeURIComponent(email)}`, client, JSON_HEADERS),
@@ -322,6 +332,10 @@ export function useClients() {
     if (!Array.isArray(emails) || emails.length === 0) return Promise.resolve(null);
     return bulkAdjustMut.mutateAsync({ emails, addDays, addBytes });
   }, [bulkAdjustMut]);
+  const bulkAssignGroup = useCallback((emails: string[], group: string) => {
+    if (!Array.isArray(emails) || emails.length === 0) return Promise.resolve(null);
+    return bulkAssignGroupMut.mutateAsync({ emails, group });
+  }, [bulkAssignGroupMut]);
   const attach = useCallback((email: string, inboundIds: number[]) => {
     if (!email) return Promise.resolve(null as unknown as Msg<unknown>);
     return attachMut.mutateAsync({ email, inboundIds });
@@ -407,6 +421,7 @@ export function useClients() {
     total,
     filtered,
     summary,
+    allGroups,
     hydrate,
     query,
     setQuery,
@@ -427,6 +442,7 @@ export function useClients() {
     remove,
     bulkDelete,
     bulkAdjust,
+    bulkAssignGroup,
     attach,
     detach,
     resetTraffic,

frontend/src/hooks/usePageTitle.ts

@@ -6,6 +6,7 @@ const TITLE_KEYS: Record<string, string> = {
   '/': 'menu.dashboard',
   '/inbounds': 'menu.inbounds',
   '/clients': 'menu.clients',
+  '/groups': 'menu.groups',
   '/nodes': 'menu.nodes',
   '/settings': 'menu.settings',
   '/xray': 'menu.xray',