Add folder support: keys that open sub-pages with a back key

A key with `folder: NAME` swaps the deck to the keys defined under
`folders.NAME`. Every folder shows a back key (slot 0 by default, drawn
with a built-in chevron) that pops back to the parent; folders nest.

The daemon now runs one page at a time: per-key setup moved into
setupKey(), openPage() draws a page under its own context, and
page.stop() tears it down before the next page is drawn (and on exit,
so device death no longer waits on uncancelled goroutines).

streamdeck-ctl gains `key set -folder`, reports kind "folder", lists
every folder in status, and refuses to `key press` a navigation-only
key. The Omarchy grid shows a folder glyph.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012ARLa4moGb154fzc9TevgV

omarchy-plugin/Model.js

@@ -123,6 +123,7 @@ function formatDuration(seconds) {
 // from streamdeck-ctl, so this list tracks KeyStatus.Kind in status.go.
 function kindGlyph(kind) {
   switch (kind) {
+  case "folder": return "󰉋"  // folder
   case "toggle": return "󰄣"  // toggle-switch
   case "module": return "󰅵"  // puzzle piece
   case "text":   return "󰉼"  // text