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

README.md

@@ -352,6 +352,45 @@ keys:
     command: ""
 ```
 
+### Folders
+
+A key can open a **folder** — a second page of keys that replaces the deck until
+you go back. Folders nest to any depth, and every folder gets a back key (slot 0
+by default, drawn with a built-in arrow). Pressing the folder key's `command`, if
+it has one, still runs.
+
+```yaml
+keys:
+  12:
+    icon: folder.png
+    text: Media
+    folder: media          # name of an entry under folders:
+
+folders:
+  media:
+    back:                  # optional — defaults to key 0, built-in arrow, "Back"
+      key: 0
+      icon: back.png       # omit to use the built-in arrow
+      text: Back
+    keys:                  # same key types as the root page: icons, GIFs,
+      1:                   # text, toggles, modules, live text, priv: commands
+        icon: play.png
+        command: playerctl play-pause
+      2:
+        icon: more.png
+        folder: media-extra   # folders can open other folders
+  media-extra:
+    keys:
+      1:
+        text: Next
+        command: playerctl next
+```
+
+Each page's animations, polls, and live-text refreshes are stopped when you leave
+it and restarted when you return. A config reload always lands on the root page.
+Folder keys show up in `streamdeck-ctl status` with kind `folder`, and every
+folder's keys are listed under `folders`.
+
 ### Status / toggle keys
 
 A key can poll any shell command on an interval and show one of two icons based on the result. Pressing the button runs `command` as usual, and the icon re-checks ~400 ms later so it reflects the new state immediately.