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/ClientFormModal.tsx

@@ -1,6 +1,7 @@
 import { useEffect, useMemo, useState } from 'react';
 import { useTranslation } from 'react-i18next';
 import {
+  AutoComplete,
   Button,
   Col,
   Form,
@@ -61,6 +62,7 @@ interface ClientFormModalProps {
   attachedIds?: number[];
   ipLimitEnable?: boolean;
   tgBotEnable?: boolean;
+  groups?: string[];
   save: (
     payload: Record<string, unknown> | SaveCreatePayload,
     meta: SaveMetaEdit | SaveMetaCreate,
@@ -83,6 +85,7 @@ interface FormState {
   reset: number;
   limitIp: number;
   tgId: number;
+  group: string;
   comment: string;
   enable: boolean;
   inboundIds: number[];
@@ -104,6 +107,7 @@ function emptyForm(): FormState {
     reset: 0,
     limitIp: 0,
     tgId: 0,
+    group: '',
     comment: '',
     enable: true,
     inboundIds: [],
@@ -128,6 +132,7 @@ export default function ClientFormModal({
   attachedIds = [],
   ipLimitEnable = false,
   tgBotEnable = false,
+  groups = [],
   save,
   onOpenChange,
 }: ClientFormModalProps) {
@@ -163,6 +168,7 @@ export default function ClientFormModal({
         reset: Number(client.reset) || 0,
         limitIp: client.limitIp || 0,
         tgId: Number(client.tgId) || 0,
+        group: client.group || '',
         comment: client.comment || '',
         enable: !!client.enable,
         inboundIds: Array.isArray(attachedIds) ? [...attachedIds] : [],
@@ -287,6 +293,7 @@ export default function ClientFormModal({
       reset: form.reset,
       limitIp: form.limitIp,
       tgId: form.tgId,
+      group: form.group,
       comment: form.comment,
       enable: form.enable,
       inboundIds: form.inboundIds,
@@ -507,6 +514,21 @@ export default function ClientFormModal({
                 <Input value={form.comment} onChange={(e) => update('comment', e.target.value)} />
               </Form.Item>
             </Col>
+            <Col xs={24} md={12}>
+              <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>
+            </Col>
           </Row>
 
           <Form.Item label={t('pages.clients.attachedInbounds')} required={!isEdit}>