feat(admin): redesign snippets master-detail with collapsible groups and metadata popover

- SnippetList: collapsible folder groups, compact rows with colored type icons,
  hover external link + delete actions, client-side search
- SnippetsRouteViewContent: lazy per-group fetch via getSnippetGroups +
  getGroupSnippets, FocusScope + useListKeyboard (Case A), header overflow
  menu for Import/Update Deps, search input replaces type/group selects
- SnippetEditor: editor-primary layout — CodeEditor fills body, metadata
  fields move to new SnippetMetaPopover (Settings popover) with sectioned
  rhythm, iconOnly header actions
- SnippetPrimitives.Field: switch label+space-y-1 to div+grid gap-1.5 for
  consistent baseline with TextInput's labeled variant
- i18n: add snippets.list.{searchPlaceholder,emptyGroup,noResults,openExternal},
  snippets.editor.action.settings, snippets.filter.groupAria
- Drop unused snippetPageSize, readSnippetTypeFilter, groupSnippetList

Author: Innei