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/styles/page-shell.css

@@ -4,6 +4,7 @@
 .xray-page,
 .settings-page,
 .nodes-page,
+.groups-page,
 .api-docs-page {
   --bg-page: #e6e8ec;
   --bg-card: #ffffff;
@@ -17,6 +18,7 @@
 .xray-page.is-dark,
 .settings-page.is-dark,
 .nodes-page.is-dark,
+.groups-page.is-dark,
 .api-docs-page.is-dark {
   --bg-page: #1a1b1f;
   --bg-card: #23252b;
@@ -28,6 +30,7 @@
 .xray-page.is-dark.is-ultra,
 .settings-page.is-dark.is-ultra,
 .nodes-page.is-dark.is-ultra,
+.groups-page.is-dark.is-ultra,
 .api-docs-page.is-dark.is-ultra {
   --bg-page: #000;
   --bg-card: #101013;
@@ -45,6 +48,8 @@
 .settings-page .ant-layout-content,
 .nodes-page .ant-layout,
 .nodes-page .ant-layout-content,
+.groups-page .ant-layout,
+.groups-page .ant-layout-content,
 .api-docs-page .ant-layout,
 .api-docs-page .ant-layout-content {
   background: transparent;
@@ -56,6 +61,7 @@
 .xray-page .content-shell,
 .settings-page .content-shell,
 .nodes-page .content-shell,
+.groups-page .content-shell,
 .api-docs-page .content-shell {
   background: transparent;
 }
@@ -65,14 +71,16 @@
 .inbounds-page .content-area,
 .xray-page .content-area,
 .settings-page .content-area,
-.nodes-page .content-area {
+.nodes-page .content-area,
+.groups-page .content-area {
   padding: 24px;
 }
 
 @media (max-width: 768px) {
   .clients-page .content-area,
   .inbounds-page .content-area,
-  .nodes-page .content-area {
+  .nodes-page .content-area,
+  .groups-page .content-area {
     padding: 8px;
   }
 }
@@ -130,14 +138,16 @@
 
 .clients-page .summary-card,
 .inbounds-page .summary-card,
-.nodes-page .summary-card {
+.nodes-page .summary-card,
+.groups-page .summary-card {
   padding: 16px;
 }
 
 @media (max-width: 768px) {
   .clients-page .summary-card,
   .inbounds-page .summary-card,
-  .nodes-page .summary-card {
+  .nodes-page .summary-card,
+  .groups-page .summary-card {
     padding: 8px;
   }
 }