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/pages/clients/ClientBulkAddModal.tsx

@@ -1,6 +1,6 @@
 import { useEffect, useMemo, useState } from 'react';
 import { useTranslation } from 'react-i18next';
-import { Button, Form, Input, InputNumber, Modal, Select, Space, Switch, message } from 'antd';
+import { AutoComplete, Button, Form, Input, InputNumber, Modal, Select, Space, Switch, message } from 'antd';
 import { ReloadOutlined } from '@ant-design/icons';
 import dayjs from 'dayjs';
 import type { Dayjs } from 'dayjs';
@@ -21,6 +21,7 @@ interface ClientBulkAddModalProps {
   open: boolean;
   inbounds: InboundOption[];
   ipLimitEnable?: boolean;
+  groups?: string[];
   onOpenChange: (open: boolean) => void;
   onSaved?: () => void;
 }
@@ -36,6 +37,7 @@ function emptyForm(): FormState {
     emailPostfix: '',
     quantity: 1,
     subId: '',
+    group: '',
     comment: '',
     flow: '',
     limitIp: 0,
@@ -50,6 +52,7 @@ export default function ClientBulkAddModal({
   open,
   inbounds,
   ipLimitEnable = false,
+  groups = [],
   onOpenChange,
   onSaved,
 }: ClientBulkAddModalProps) {
@@ -157,6 +160,7 @@ export default function ClientBulkAddModal({
           expiryTime: form.expiryTime,
           reset: Number(form.reset) || 0,
           limitIp: Number(form.limitIp) || 0,
+          group: form.group,
           comment: form.comment,
           enable: true,
         },
@@ -263,6 +267,20 @@ export default function ClientBulkAddModal({
             </Space.Compact>
           </Form.Item>
 
+          <Form.Item label={t('pages.clients.group')} tooltip={t('pages.clients.groupDesc')}>
+            <AutoComplete
+              value={form.group}
+              placeholder={t('pages.clients.groupPlaceholder')}
+              options={groups.map((g) => ({ value: g }))}
+              onChange={(v) => update('group', v ?? '')}
+              filterOption={(input, option) =>
+                String(option?.value ?? '').toLowerCase().includes((input || '').toLowerCase())
+              }
+              allowClear
+              style={{ width: '100%' }}
+            />
+          </Form.Item>
+
           <Form.Item label={t('comment')}>
             <Input value={form.comment} onChange={(e) => update('comment', e.target.value)} />
           </Form.Item>