Sales Channels: Optional Handles

Sales channels now carry an optional handle — a short, stable identifier that travels with the channel even if the display name changes. You can set a handle when creating a channel, and edit it on the channel header card; it auto-suggests a kebab-case version of the channel name and you can override it. Handles are lowercased, validated against the allowed shape, and must be unique inside your company; the same handle can be reused across companies.