Add mailbox/email UI translations

Add localization entries for mailbox integration and admin mailbox settings. Adds admin.settings.mailbox (and gmail note) plus mailbox-related UI strings (connect, connectGmail, cancel, copySender/Subject, displayName & hints, file type/label filters, loadingMore, save, etc.) and registers "mailbox" in system settings list to enable UI for email attachment import and configuration.

frontend/editor/public/locales/en-US/translation.toml

@@ -576,6 +576,13 @@ success = "Settings saved successfully"
 title = "Admin Settings"
 workspace = "Workspace"
 
+[admin.settings.mailbox]
+description = "Configure the OAuth connection used to read mailbox attachments."
+title = "Mailbox"
+
+[admin.settings.mailbox.gmail]
+note = "These values are read from settings.yml under mailbox.gmail. Changes require a server restart."
+
 [admin.settings.advanced]
 description = "Configure advanced features and experimental functionality."
 features = "Feature Flags"
@@ -3697,13 +3704,22 @@ title = "Open the tool to start editing"
 accounts = "Accounts"
 attachments = "Attachments"
 back = "Back"
+cancel = "Cancel"
 cacheHint = "Message metadata is cached locally."
 cacheReady = "Local cache active"
 connectAccount = "Connect account"
 connectDescription = "Connect your email account to securely transfer attachments into your PDF workflow."
 connectTitle = "Connect mailbox"
+connectGmail = "Connect Gmail"
+closeConnectDialog = "Close"
+copySender = "Copy sender"
+copySubject = "Copy subject"
+customFileType = "Enter a custom file type and press Enter"
 demoBody = "Attachments can be transferred directly into the Stirling PDF workspace after download."
 download = "Import"
+displayName = "Display name"
+displayNameHint = "This name is displayed instead of the email address in the mailbox."
+displayNamePlaceholder = "e.g. Peter Example"
 eyebrow = "File sources"
 folders = "Mailbox"
 inbox = "Inbox"
@@ -3720,6 +3736,10 @@ oauthNote = "Sign-in uses OAuth. Passwords are not stored by Stirling."
 queued = "Queued"
 refresh = "Refresh inbox"
 searchPlaceholder = "Search messages"
+save = "Save"
+fileTypeFilter = "Filter by file type"
+labelFilter = "Filter by labels"
+loadingMore = "Loading more messages ..."
 selectMessage = "Select a message"
 selectMessageHint = "Choose an email from the list."
 settings = "Email settings"
@@ -9761,6 +9781,7 @@ database = "Database"
 endpoints = "Endpoints"
 features = "Features"
 folderAccess = "Folder Access"
+mailbox = "Mailbox"
 mcp = "MCP Server"
 storageSharing = "File Storage & Sharing"
 systemSettings = "System Settings"