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

cmd/streamdeck-ctl/main.go

@@ -78,6 +78,7 @@ key set flags (only the flags you pass are written):
   -icon NAME            -icon-true NAME       -icon-false NAME
   -text STR             -text-color STR       -text-command STR      -refresh DUR
   -command STR          -module NAME          -function NAME
+  -folder NAME          (pressing the key opens that folder)
   -param K=V            (repeatable)
   -poll-command STR     -poll-match STR       -poll-interval DUR
   -unset FIELD          (repeatable — removes a field)
@@ -161,6 +162,7 @@ func keySet(cfgPath string, index int, args []string) error {
 		"text-command":  fs.String("text-command", "", "command whose stdout becomes the overlay text"),
 		"refresh":       fs.String("refresh", "", "how often to re-run text-command, e.g. 1s"),
 		"command":       fs.String("command", "", "shell command to run on press"),
+		"folder":        fs.String("folder", "", "folder (from the folders: block) the key opens"),
 		"module":        fs.String("module", "", "module name from modules.yaml"),
 		"function":      fs.String("function", "", "function name within the module"),
 		"poll-command":  fs.String("poll-command", "", "command used to poll toggle state"),
@@ -171,6 +173,7 @@ func keySet(cfgPath string, index int, args []string) error {
 		"icon": "icon", "icon-true": "icon_true", "icon-false": "icon_false",
 		"text": "text", "text-color": "text_color", "text-command": "text_command",
 		"refresh": "refresh", "command": "command", "module": "module", "function": "function",
+		"folder": "folder",
 	}
 
 	var params stringList
@@ -300,6 +303,10 @@ func keyPress(cfgPath string, index int) error {
 		return fmt.Errorf("key %d is not configured", index)
 	}
 
+	if key.Folder != "" && key.Command == "" && key.Module == "" {
+		return fmt.Errorf("key %d opens folder %q — navigation only happens on the deck", index, key.Folder)
+	}
+
 	command := key.Command
 	if key.Module != "" && key.Function != "" {
 		reg, err := modules.LoadRegistry(config.ModulesPath(cfgPath))