fix(plugin): DisablePlugin 禁止禁用未安装插件，防止脏写 disabled_plugins

问题：POST /api/v1/plugins/<name>/disable 对不存在的插件也会把它写进
disabled_plugins 表（registry.go DisablePlugin 无条件 AddDisabledPlugin），
产生脏数据堆积，且同名插件日后真实安装会被误判为已禁用。

修复：
- registry.go 新增 pluginInstalled(name)：已加载 / 已注册工厂(内置) / 插件目录存在，
  任一命中视为已安装
- DisablePlugin 开头校验：未安装返回 'plugin X not installed'，不写 disabled_plugins
- webui handler：未安装→404，已禁用→409（原都返回500）；enable 失败含'failed'→404
- 新增 registry_disable_test.go：覆盖未安装拒绝/内置判定/目录存在判定/普通文件不算

本机端到端验证：disable 不存在插件返回404且表无脏数据；真实插件 disable/enable 正常

internal/plugin/registry_disable_test.go

@@ -0,0 +1,74 @@
+package plugin
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	sdk "gitcode.com/JianFeeeee/HomeAgent/internal/sdk"
+)
+
+// TestDisablePluginNotInstalled 验证 DisablePlugin 对未安装插件的拒绝：
+// 修复前会把任意名字写进 disabled_plugins 表（脏数据），修复后应直接报错。
+func TestDisablePluginNotInstalled(t *testing.T) {
+	r := &Registry{
+		plugins:       make(map[string]sdk.Plugin),
+		factories:     make(map[string]NativeFactory),
+		knownDisabled: make(map[string]bool),
+		plgDir:        t.TempDir(),
+	}
+	err := r.DisablePlugin("nonexistent_test_xyz", "test")
+	if err == nil {
+		t.Fatal("DisablePlugin should reject a plugin that is not installed")
+	}
+	if err.Error() != "plugin nonexistent_test_xyz not installed" {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if r.knownDisabled["nonexistent_test_xyz"] {
+		t.Fatal("knownDisabled must not contain a non-installed plugin")
+	}
+}
+
+// TestPluginInstalledFactory 内置插件（工厂注册，无物理目录）应视为已安装。
+func TestPluginInstalledFactory(t *testing.T) {
+	r := &Registry{
+		plugins:       make(map[string]sdk.Plugin),
+		factories:     map[string]NativeFactory{"builtin_demo": nil},
+		knownDisabled: make(map[string]bool),
+		plgDir:        t.TempDir(),
+	}
+	if !r.pluginInstalled("builtin_demo") {
+		t.Fatal("factory-registered builtin plugin should be considered installed")
+	}
+	if r.pluginInstalled("no_such_plugin") {
+		t.Fatal("unknown plugin should not be considered installed")
+	}
+}
+
+// TestPluginInstalledDir 外部插件按 plgDir/<name> 目录存在判定已安装。
+func TestPluginInstalledDir(t *testing.T) {
+	dir := t.TempDir()
+	if err := os.MkdirAll(filepath.Join(dir, "external_demo"), 0o755); err != nil {
+		t.Fatalf("mkdir: %v", err)
+	}
+	r := &Registry{
+		plugins:       make(map[string]sdk.Plugin),
+		factories:     make(map[string]NativeFactory),
+		knownDisabled: make(map[string]bool),
+		plgDir:        dir,
+	}
+	if !r.pluginInstalled("external_demo") {
+		t.Fatal("plugin with an existing directory should be considered installed")
+	}
+	// 同名普通文件（非目录）不算已安装
+	if err := os.WriteFile(filepath.Join(dir, "just_a_file"), []byte("x"), 0o644); err != nil {
+		t.Fatalf("write: %v", err)
+	}
+	if r.pluginInstalled("just_a_file") {
+		t.Fatal("a regular file must not count as an installed plugin")
+	}
+	// 空名兜底
+	if r.pluginInstalled("") {
+		t.Fatal("empty plugin name must not be considered installed")
+	}
+}