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

internal/config/edit.go

@@ -103,7 +103,7 @@ var fieldOrder = []string{
 	"icon", "icon_true", "icon_false",
 	"text", "text_color", "text_command", "refresh",
 	"module", "function", "params",
-	"command", "poll",
+	"command", "folder", "poll",
 }
 
 func sortedFieldNames(m map[string]any) []string {