feat: add calendar, kanban, chat, file manager, and projects apps

Adds five "app" pages that move AdminLTE closer to feature parity with
Tabler and CoreUI Pro:

- pages/calendar.html      FullCalendar 6 integration (CDN). Month /
                           week / day / list views, drag-and-drop from
                           an external events sidebar, click-to-create,
                           click-to-delete. MIT-licensed, no jQuery.
- pages/kanban.html        SortableJS-powered board with four lanes.
                           Drag cards within and between lanes,
                           live-updating count badges, ghost preview
                           during drag, prompt-driven "add card" per
                           lane.
- pages/chat.html          Two-pane full-page chat. Contact list with
                           online-status dots and unread badges,
                           scrollable message thread with sent /
                           received bubbles, live composer that appends
                           real messages with timestamps.
- pages/file-manager.html  Folder tree (Bootstrap list-group flush) +
                           breadcrumb + storage quota. Grid and list
                           views with a toggle, file-type icons in
                           semantic colors, shared-file badge.
- pages/projects.html      Four KPI summary cards, full data table with
                           status badges, bucketed progress bars,
                           stacked team avatars, priority chips, and
                           pagination.

External libraries are CDN-loaded so nothing is added to the framework
bundle. Sidenav entries land in a later commit.