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/config.go

@@ -42,6 +42,34 @@ type KeyConfig struct {
 	Module   string            `yaml:"module"`
 	Function string            `yaml:"function"`
 	Params   map[string]string `yaml:"params"`
+
+	// Folder key: pressing it swaps the deck to the named folder's keys
+	// (defined under the top-level `folders:` block). Folders nest — a folder's
+	// key may itself have a folder — and every folder shows a back key.
+	Folder string `yaml:"folder"`
+}
+
+// BackConfig customises the back key every folder shows. All fields are
+// optional: the default is key 0 drawn with a built-in arrow and "Back" label.
+type BackConfig struct {
+	Key       *int   `yaml:"key"`        // slot index for the back key (default 0)
+	Icon      string `yaml:"icon"`       // icon filename relative to icons_dir (default: built-in arrow)
+	Text      string `yaml:"text"`       // label (default "Back")
+	TextColor string `yaml:"text_color"` // same values as KeyConfig.TextColor
+}
+
+// FolderConfig is a sub-page of keys shown while a folder is open.
+type FolderConfig struct {
+	Back BackConfig        `yaml:"back"`
+	Keys map[int]KeyConfig `yaml:"keys"`
+}
+
+// BackKey returns the slot index the folder's back key occupies.
+func (f FolderConfig) BackKey() int {
+	if f.Back.Key != nil {
+		return *f.Back.Key
+	}
+	return 0
 }
 
 // DefaultConfigPath returns the default config file path, respecting XDG_CONFIG_HOME.
@@ -65,6 +93,9 @@ type Config struct {
 	Brightness int               `yaml:"brightness"`
 	Device     DeviceConfig      `yaml:"device"`
 	Keys       map[int]KeyConfig `yaml:"keys"`
+	// Folders are sub-pages opened by keys with a `folder:` field. The root
+	// page is Keys; a folder's keys replace it until its back key is pressed.
+	Folders map[string]FolderConfig `yaml:"folders"`
 }
 
 // DeviceConfig allows overriding USB IDs (defaults work for Stream Deck XL v2).