feat(folders): drag-and-drop reorder for all folders

[shukiv]

What

Adds drag-and-drop reordering of folders in Settings → Folders ("Your Folders"). Each row gets a drag handle; dropping persists the new order via JMAP Mailbox/set { sortOrder }, and the sidebar reflects it immediately.

Reorders all folders (standard roles included), within their sibling group — it reorders, it does not reparent (dropping onto another parent's folder is ignored).

How

• lib/utils.ts (buildMailboxTree / sortNodes): promote sortOrder to the primary sort key. Untouched folders keep sortOrder: 0, so the default arrangement (role priority → year → name) is unchanged until the user drags something. Sidebar + settings share this builder, so both respect the order.
• stores/email-store.ts: new reorderMailboxes(client, orderedIds) — assigns 1-based sortOrder to a sibling group, optimistic local update, persists each via updateMailbox, re-syncs on failure. Handles shared-account (viewingAccountId) views.
• components/settings/folder-settings.tsx: @dnd-kit DnD context; a SortableContext per sibling group; a SortableFolderRow handle wrapper. Pointer sensor with a 5px activation distance so the row's existing buttons still work; keyboard sensor for a11y.
• New deps: @dnd-kit/core, @dnd-kit/sortable, @dnd-kit/utilities.
• i18n: settings.folders.reorder / reorder_error (en + he).

Testing

• Drag a folder within its level → order persists, sidebar updates, survives reload
• Default (nothing dragged) order unchanged
• Row buttons (rename/delete/create-subfolder/icon) still click without starting a drag
• Keyboard: focus handle → space + arrows reorders
• Drop onto a different parent's folder → ignored (no reparent)
• tsc clean; lint clean (no new warnings)