Function BmsItemSheetMixin

Mixin that eliminates boilerplate shared across all BMS item sheets:

• _prepareContext seeds context.tabs/item/system/flags/config
• _preparePartContext default (context.tab = context.tabs[partId])
• _getHeaderControls unshifts Edit Image + Snapshots buttons for owners
• _onEditImage / _onManageSnapshots static action handlers
• _onDropActiveEffect standard guard

The host sheet must:

• include editImage: SheetClass._onEditImage in DEFAULT_OPTIONS.actions
• call await super._prepareContext(options) and use the returned context

Sheets with custom _preparePartContext logic (effect-sheet, journal pages) should NOT use this mixin or should override _preparePartContext.