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.

database/db.go

@@ -68,6 +68,7 @@ func initModels() error {
 		&model.ApiToken{},
 		&model.ClientRecord{},
 		&model.ClientInbound{},
+		&model.ClientGroup{},
 		&model.InboundFallback{},
 	}
 	for _, mdl := range models {

database/model/model.go

@@ -371,6 +371,7 @@ type Client struct {
 	Enable     bool           `json:"enable" form:"enable"`         // Whether the client is enabled
 	TgID       int64          `json:"tgId" form:"tgId"`             // Telegram user ID for notifications
 	SubID      string         `json:"subId" form:"subId"`           // Subscription identifier
+	Group      string         `json:"group,omitempty" form:"group"` // Logical grouping label
 	Comment    string         `json:"comment" form:"comment"`       // Client comment
 	Reset      int            `json:"reset" form:"reset"`           // Reset period in days
 	CreatedAt  int64          `json:"created_at,omitempty"`         // Creation timestamp
@@ -392,6 +393,7 @@ type ClientRecord struct {
 	ExpiryTime int64  `json:"expiryTime" gorm:"column:expiry_time"`
 	Enable     bool   `json:"enable" gorm:"default:true"`
 	TgID       int64  `json:"tgId" gorm:"column:tg_id"`
+	Group      string `json:"group" gorm:"column:group_name;default:''"`
 	Comment    string `json:"comment"`
 	Reset      int    `json:"reset" gorm:"default:0"`
 	CreatedAt  int64  `json:"createdAt" gorm:"autoCreateTime:milli"`
@@ -400,6 +402,15 @@ type ClientRecord struct {
 
 func (ClientRecord) TableName() string { return "clients" }
 
+type ClientGroup struct {
+	Id        int    `json:"id" gorm:"primaryKey;autoIncrement"`
+	Name      string `json:"name" gorm:"uniqueIndex;not null"`
+	CreatedAt int64  `json:"createdAt" gorm:"autoCreateTime:milli"`
+	UpdatedAt int64  `json:"updatedAt" gorm:"autoUpdateTime:milli"`
+}
+
+func (ClientGroup) TableName() string { return "client_groups" }
+
 // MarshalJSON emits the reverse column as a nested JSON object rather than an
 // escaped JSON-text string, matching the same convention Inbound uses for its
 // JSON-text columns. Empty storage renders as null.
@@ -472,6 +483,7 @@ func (c *Client) ToRecord() *ClientRecord {
 		ExpiryTime: c.ExpiryTime,
 		Enable:     c.Enable,
 		TgID:       c.TgID,
+		Group:      c.Group,
 		Comment:    c.Comment,
 		Reset:      c.Reset,
 		CreatedAt:  c.CreatedAt,
@@ -499,6 +511,7 @@ func (r *ClientRecord) ToClient() *Client {
 		ExpiryTime: r.ExpiryTime,
 		Enable:     r.Enable,
 		TgID:       r.TgID,
+		Group:      r.Group,
 		Comment:    r.Comment,
 		Reset:      r.Reset,
 		CreatedAt:  r.CreatedAt,
@@ -623,6 +636,12 @@ func MergeClientRecord(existing *ClientRecord, incoming *ClientRecord) []ClientM
 			existing.Comment = incoming.Comment
 		}
 	}
+	if existing.Group != incoming.Group && incoming.Group != "" {
+		if incomingNewer || existing.Group == "" {
+			keep("group", existing.Group, incoming.Group, incoming.Group)
+			existing.Group = incoming.Group
+		}
+	}
 	if existing.Enable != incoming.Enable {
 		if incoming.Enable {
 			if !existing.Enable {