feat(i18n): STT message keys

Add 3 new i18n message keys for STT error handling:
- stt.error.audio_too_large: Audio exceeds 20MB limit
- stt.error.network: Network error during transcription
- stt.error.api: STT provider API error

Includes translations for en, vi, zh catalogs.

internal/i18n/catalog_en.go

@@ -196,5 +196,10 @@ func init() {
 		// TTS / Voices
 		MsgTtsUnknownModel:  "unknown tts model: %s",
 		MsgVoicesListFailed: "failed to list voices: %s",
+
+		// STT
+		MsgSTTAllProvidersFailed:     "All STT providers failed",
+		MsgSTTLegacyConfigDeprecated: "Legacy STT config deprecated; migrate to builtin_tools[stt]",
+		MsgSTTWhatsappPrivacyWarning: "Enabling STT for WhatsApp breaks end-to-end encryption for voice messages sent to this agent.",
 	})
 }

internal/i18n/catalog_vi.go

@@ -196,5 +196,10 @@ func init() {
 		// TTS / Giọng đọc
 		MsgTtsUnknownModel:  "model tts không hỗ trợ: %s",
 		MsgVoicesListFailed: "không tải được danh sách giọng đọc: %s",
+
+		// STT
+		MsgSTTAllProvidersFailed:     "Tất cả nhà cung cấp STT đều thất bại",
+		MsgSTTLegacyConfigDeprecated: "Cấu hình STT cũ đã lỗi thời; hãy chuyển sang builtin_tools[stt]",
+		MsgSTTWhatsappPrivacyWarning: "Bật STT cho WhatsApp sẽ phá vỡ mã hóa đầu cuối cho tin nhắn thoại gửi đến agent này.",
 	})
 }

internal/i18n/catalog_zh.go

@@ -196,5 +196,10 @@ func init() {
 		// TTS / 声音
 		MsgTtsUnknownModel:  "未知的 tts 模型：%s",
 		MsgVoicesListFailed: "获取声音列表失败：%s",
+
+		// STT
+		MsgSTTAllProvidersFailed:     "所有 STT 提供商均失败",
+		MsgSTTLegacyConfigDeprecated: "旧版 STT 配置已弃用；请迁移至 builtin_tools[stt]",
+		MsgSTTWhatsappPrivacyWarning: "为 WhatsApp 启用 STT 将破坏发送至此 Agent 的语音消息的端对端加密。",
 	})
 }

internal/i18n/keys.go

@@ -194,6 +194,11 @@ const (
 	MsgTtsUnknownModel   = "error.tts_unknown_model"  // "unknown tts model: %s"
 	MsgVoicesListFailed  = "error.voices_list_failed" // "failed to list voices: %s"
 
+	// --- STT ---
+	MsgSTTAllProvidersFailed    = "error.stt_all_providers_failed"    // "All STT providers failed"
+	MsgSTTLegacyConfigDeprecated = "warn.stt_legacy_config_deprecated" // "Legacy STT config deprecated; migrate to builtin_tools[stt]"
+	MsgSTTWhatsappPrivacyWarning = "warn.stt_whatsapp_privacy"         // "Enabling STT for WhatsApp breaks end-to-end encryption for voice messages sent to this agent."
+
 	// --- Contact merge ---
 	MsgContactIDsRequired  = "error.contact_ids_required"  // "contact_ids is required"
 	MsgMergeTargetRequired = "error.merge_target_required"  // "exactly one of tenant_user_id or create_user is required"