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/schemas/client.ts

@@ -25,6 +25,7 @@ export const ClientRecordSchema = z.object({
   expiryTime: z.number().optional(),
   limitIp: z.number().optional(),
   tgId: z.union([z.number(), z.string()]).optional(),
+  group: z.string().optional(),
   comment: z.string().optional(),
   enable: z.boolean().optional(),
   reset: z.number().optional(),
@@ -63,6 +64,7 @@ export const ClientPageResponseSchema = z.object({
   page: z.number(),
   pageSize: z.number(),
   summary: ClientsSummarySchema.nullable().optional(),
+  groups: nullableStringArray.optional(),
 });
 
 export const ClientHydrateSchema = z.object({
@@ -97,6 +99,13 @@ export const DelDepletedResultSchema = z.object({
 
 export const OnlinesSchema = nullableStringArray;
 
+export const GroupSummarySchema = z.object({
+  name: z.string(),
+  clientCount: z.number(),
+});
+
+export const GroupSummaryListSchema = z.array(GroupSummarySchema).nullable().transform((v) => v ?? []);
+
 export const ClientFormSchema = z.object({
   email: z.string().trim().min(1, 'pages.clients.email'),
   subId: z.string(),
@@ -111,6 +120,7 @@ export const ClientFormSchema = z.object({
   reset: z.number().int().min(0),
   limitIp: z.number().int().min(0),
   tgId: z.number().int().min(0),
+  group: z.string(),
   comment: z.string(),
   enable: z.boolean(),
   inboundIds: z.array(z.number()),
@@ -137,6 +147,7 @@ export const ClientBulkAddFormSchema = z.object({
   emailPostfix: z.string(),
   quantity: z.number().int().min(1).max(100),
   subId: z.string(),
+  group: z.string(),
   comment: z.string(),
   flow: z.string(),
   limitIp: z.number().int().min(0),
@@ -158,3 +169,4 @@ export type BulkCreateResult = z.infer<typeof BulkCreateResultSchema>;
 export type ClientBulkAddFormValues = z.infer<typeof ClientBulkAddFormSchema>;
 export type ClientBulkAdjustFormValues = z.infer<typeof ClientBulkAdjustFormSchema>;
 export type ClientFormValues = z.infer<typeof ClientFormSchema>;
+export type GroupSummary = z.infer<typeof GroupSummarySchema>;