[client/ui] Introduce localisation (i18n + preferences) feature packages

Adds a tray + React translation pipeline driven by a single JSON locale
tree (frontend/src/i18n/locales) embedded into the Go binary. The tray
re-renders on language switch via a Localizer that subscribes to the
preferences store.

Layout:
- client/ui/i18n: Bundle, LanguageCode, Language, errors, embedded-FS
  loader. Pure domain, no Wails/daemon deps.
- client/ui/preferences: Store + UIPreferences for user-scope UI state,
  persisted under os.UserConfigDir()/netbird/ui-preferences.json with
  atomic writes and a subscribe/broadcast channel.
- client/ui/services: thin Wails-binding facades (services.I18n,
  services.Preferences) so React sees ctx-first signatures.
- client/ui/localizer.go: tray bridge that owns the active language,
  exposes T()/StatusLabel() and re-paints the menu on prefs change.
- tray.go: every user-facing const replaced by translation keys via
  t.loc.T(...); menu rebuild + state replay on language switch.
- main.go: //go:embed all:frontend/src/i18n/locales, wires Bundle ->
  Store -> Localizer -> Wails facades in order.

Frontend API exposed via Wails bindings: I18n.Languages, I18n.Bundle,
Preferences.Get, Preferences.SetLanguage, plus the
netbird:preferences:changed event.

Includes regenerated Wails TS bindings (peers/profileswitcher/etc.
re-emitted as part of the build) and en/hu seed bundles.

client/ui/i18n/bundle_test.go

@@ -0,0 +1,156 @@
+//go:build !android && !ios && !freebsd && !js
+
+package i18n
+
+import (
+	"testing"
+	"testing/fstest"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// fakeLocales returns an in-memory FS that mirrors the real
+// frontend/src/i18n/locales layout (root-level _index.json plus
+// <code>/common.json bundles). Used by every Bundle test so we don't
+// depend on the embedded production bundles staying stable.
+func fakeLocales() fstest.MapFS {
+	return fstest.MapFS{
+		"_index.json": {Data: []byte(`{
+            "languages": [
+                {"code": "en", "displayName": "English", "englishName": "English"},
+                {"code": "hu", "displayName": "Magyar", "englishName": "Hungarian"}
+            ]
+        }`)},
+		"en/common.json": {Data: []byte(`{
+            "tray.menu.connect": "Connect",
+            "tray.menu.installVersion": "Install version {version}",
+            "notify.update.body": "NetBird {version} is available."
+        }`)},
+		"hu/common.json": {Data: []byte(`{
+            "tray.menu.connect": "Csatlakozás",
+            "tray.menu.installVersion": "{version} telepítése"
+        }`)},
+	}
+}
+
+func TestBundle_LoadsAllLanguages(t *testing.T) {
+	b, err := NewBundle(fakeLocales())
+	require.NoError(t, err)
+
+	langs := b.Languages()
+	require.Len(t, langs, 2)
+	codes := []LanguageCode{langs[0].Code, langs[1].Code}
+	assert.ElementsMatch(t, []LanguageCode{"en", "hu"}, codes, "Languages should list every bundle that loaded")
+}
+
+func TestBundle_TranslateLooksUpKey(t *testing.T) {
+	b, err := NewBundle(fakeLocales())
+	require.NoError(t, err)
+
+	assert.Equal(t, "Csatlakozás", b.Translate("hu", "tray.menu.connect"))
+	assert.Equal(t, "Connect", b.Translate("en", "tray.menu.connect"))
+}
+
+func TestBundle_TranslateSubstitutesPlaceholders(t *testing.T) {
+	b, err := NewBundle(fakeLocales())
+	require.NoError(t, err)
+
+	assert.Equal(t, "Install version 1.2.3",
+		b.Translate("en", "tray.menu.installVersion", "version", "1.2.3"),
+		"placeholders should substitute by name")
+	assert.Equal(t, "1.2.3 telepítése",
+		b.Translate("hu", "tray.menu.installVersion", "version", "1.2.3"))
+}
+
+func TestBundle_TranslateFallsBackToEnglish(t *testing.T) {
+	b, err := NewBundle(fakeLocales())
+	require.NoError(t, err)
+
+	// notify.update.body is missing from the hu bundle; English fallback
+	// applies so the user always sees a populated label rather than the
+	// raw key.
+	got := b.Translate("hu", "notify.update.body", "version", "9.9.9")
+	assert.Equal(t, "NetBird 9.9.9 is available.", got, "missing hu key should fall back to en bundle")
+}
+
+func TestBundle_TranslateUnknownKeyReturnsKey(t *testing.T) {
+	b, err := NewBundle(fakeLocales())
+	require.NoError(t, err)
+
+	assert.Equal(t, "tray.missing", b.Translate("en", "tray.missing"),
+		"unknown key should return the key itself for debugability")
+}
+
+func TestBundle_BundleForReturnsCopy(t *testing.T) {
+	b, err := NewBundle(fakeLocales())
+	require.NoError(t, err)
+
+	m, err := b.BundleFor("en")
+	require.NoError(t, err)
+	require.NotEmpty(t, m, "BundleFor should return populated map for known language")
+
+	m["tray.menu.connect"] = "Mutated"
+	assert.Equal(t, "Connect", b.Translate("en", "tray.menu.connect"),
+		"BundleFor must return a copy, not the live map")
+}
+
+func TestBundle_BundleForUnknownLanguage(t *testing.T) {
+	b, err := NewBundle(fakeLocales())
+	require.NoError(t, err)
+
+	_, err = b.BundleFor("xx")
+	assert.ErrorIs(t, err, ErrUnsupportedLanguage)
+}
+
+func TestBundle_HasLanguage(t *testing.T) {
+	b, err := NewBundle(fakeLocales())
+	require.NoError(t, err)
+
+	assert.True(t, b.HasLanguage("en"))
+	assert.True(t, b.HasLanguage("hu"))
+	assert.False(t, b.HasLanguage("de"))
+}
+
+func TestBundle_MissingDefaultBundleFails(t *testing.T) {
+	// Without an en bundle we have nothing to fall back to, so construction
+	// must hard-fail. Catches packaging accidents where someone drops the
+	// English locale.
+	fs := fstest.MapFS{
+		"_index.json":    {Data: []byte(`{"languages":[{"code":"hu","displayName":"Magyar","englishName":"Hungarian"}]}`)},
+		"hu/common.json": {Data: []byte(`{"k":"v"}`)},
+	}
+	_, err := NewBundle(fs)
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "default language")
+}
+
+func TestBundle_MissingBundleSkipsLanguage(t *testing.T) {
+	// A language declared in the index but missing its bundle file is
+	// dropped from Languages with a warning — adding a new language must
+	// be a two-step process (declare + ship), not declare-only.
+	fs := fstest.MapFS{
+		"_index.json": {Data: []byte(`{"languages":[
+            {"code":"en","displayName":"English","englishName":"English"},
+            {"code":"de","displayName":"Deutsch","englishName":"German"}
+        ]}`)},
+		"en/common.json": {Data: []byte(`{"k":"v"}`)},
+	}
+	b, err := NewBundle(fs)
+	require.NoError(t, err)
+
+	langs := b.Languages()
+	require.Len(t, langs, 1)
+	assert.Equal(t, LanguageCode("en"), langs[0].Code, "language without a bundle file must be dropped")
+	assert.False(t, b.HasLanguage("de"))
+}
+
+func TestBundle_OddPlaceholderArgsDoNotPanic(t *testing.T) {
+	b, err := NewBundle(fakeLocales())
+	require.NoError(t, err)
+
+	// Trailing dangling arg should be dropped, not panic — preserves UI
+	// stability when a caller passes an unpaired placeholder by mistake.
+	got := b.Translate("en", "tray.menu.installVersion", "version", "1.2.3", "extra")
+	assert.Equal(t, "Install version 1.2.3", got)
+}