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/folders_test.go

@@ -0,0 +1,82 @@
+package config
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestLoadFolders(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "config.yaml")
+	src := `
+keys:
+  3:
+    icon: folder.png
+    folder: media
+folders:
+  media:
+    back:
+      key: 7
+      icon: back.png
+      text: Up
+    keys:
+      0:
+        icon: play.png
+        command: play
+      1:
+        icon: more.png
+        folder: nested
+  nested:
+    keys:
+      1:
+        text: Deep
+        command: echo deep
+`
+	if err := os.WriteFile(path, []byte(src), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	cfg, err := Load(path)
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if got := cfg.Keys[3].Folder; got != "media" {
+		t.Errorf("key 3 folder = %q, want media", got)
+	}
+	media, ok := cfg.Folders["media"]
+	if !ok {
+		t.Fatal("folder media missing")
+	}
+	if media.BackKey() != 7 || media.Back.Icon != "back.png" || media.Back.Text != "Up" {
+		t.Errorf("media back = %+v (key %d)", media.Back, media.BackKey())
+	}
+	if media.Keys[0].Command != "play" || media.Keys[1].Folder != "nested" {
+		t.Errorf("media keys = %+v", media.Keys)
+	}
+	nested := cfg.Folders["nested"]
+	if nested.BackKey() != 0 {
+		t.Errorf("nested back key = %d, want default 0", nested.BackKey())
+	}
+	if nested.Keys[1].Text != "Deep" {
+		t.Errorf("nested keys = %+v", nested.Keys)
+	}
+}
+
+func TestApplyKeyEditFolder(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "config.yaml")
+	if err := os.WriteFile(path, []byte("keys: {}\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	err := ApplyKeyEdit(path, 5, KeyEdit{Set: map[string]any{"icon": "f.png", "folder": "media"}})
+	if err != nil {
+		t.Fatalf("ApplyKeyEdit: %v", err)
+	}
+	cfg, err := Load(path)
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if cfg.Keys[5].Folder != "media" || cfg.Keys[5].Icon != "f.png" {
+		t.Errorf("key 5 = %+v", cfg.Keys[5])
+	}
+}