Add sidebar project multi-select with group drag and context menu

[MaplumeX]

Summary

Aligns the sidebar with Things 3 Mac: project rows can now be multi-selected, dragged as a group, and acted on as a group via the context menu. Area rows are not part of the selection.

Changes

New selection store (@taskora/api)

• sidebarSelection.store.ts: ids (in selection order), anchorId, and order (display order of sidebar project rows, registered by the sidebar).
  • toggle(id, currentId) — ⌘/Ctrl+click; on the first multi-select, the currently open project is included.
  • extendTo(id, currentId) — ⇧+click; selects the contiguous range from the anchor (defaults to the current project).
  • sidebarSelectionGroup() — returns the group in display order, or null when fewer than 2 rows are selected.
  • sidebarContextMenuTargets(id) — right-clicking inside the selection targets the whole group; otherwise the selection is cleared and only that row is targeted.
• Window-local only; independent from keyboard Selection and touch Multi-Select Mode.

Sidebar behavior (SidebarProjectSection, ProjectItem)

• Modifier-clicks only change the selection, never navigate; ⇧+click also suppresses native text selection. Plain clicks clear the selection and navigate.
• Highlight follows the selection while it is active, otherwise it follows the current route. The selection is cleared on route change.
• Dragging a selected project drags the whole group: the other members collapse after the drag-start commit (via useCollapseAfterDragStart), the drag overlay shows a DragCountBadge, and on drop the group replaces the dragged project in display order (expandProjectGroup).
• Ownership changes are written per project; the order is written once all of them succeed. Any failure rolls back and reports a single toast. Cancelling restores the members. The selection is preserved.
• Dragging a project outside the selection drags only that project.

Group context menu (ProjectContextMenu)

• Right-clicking a selected project opens a group menu that shows the item count and takes a snapshot of the targets when opened.
• Group actions: complete / uncomplete (uncomplete when all are completed, otherwise complete only the not-yet-completed ones; projects with remaining tasks are asked about in a single merged dialog), scheduled date, due date, tags (tri-state, applied per project on top of existing tags), move (without marking the current location), and delete (clears the selection afterwards).
• Repeat, skip, duplicate, and open-in-new-window remain single-project only.

Other

• sidebarProjectLayout: removeProjects and expandProjectGroup helpers for group drag.
• useProjectCompletion: new toggleGroup plus a group-aware remaining-tasks dialog.
• ProjectMovePicker accepts a partial current so group moves don't highlight a location.
• New i18n strings for the group completion dialog (en/zh) and the new store exported from the API package.
• CONTEXT.md documents the new Sidebar Selection domain concept; spec at .scratch/sidebar-multi-select/spec.md.

Tests

• sidebarSelection.store.test.ts: toggle/extend/group/context-menu targeting.
• SidebarProjectSection.test.tsx: group drag (collapse, ordered drop, per-project ownership writes, reorder after all writes), cancel restore, and dragging a non-selected project.
• sidebarProjectLayout.test.ts: removeProjects / expandProjectGroup.
• ProjectContextMenu.test.tsx: group menu item count, hidden single-project actions, group delete, grouped completion dialog, and group move.

Out of scope

• Multi-select for area rows.
• Keyboard (⌫ / Delete) deletion of the group.
• Dropping the group on other sidebar targets (Today, Trash, etc.).