feat: add secrets management via system keyring

SecretsManager using keyring crate (linux-native/libsecret). Store/get/
delete/list with __bterminal_keys__ metadata tracking. SettingsTab
Secrets section. No plaintext fallback.

v2/src-tauri/src/commands/secrets.rs

@@ -0,0 +1,34 @@
+use crate::secrets::SecretsManager;
+
+#[tauri::command]
+pub fn secrets_store(key: String, value: String) -> Result<(), String> {
+    SecretsManager::store_secret(&key, &value)
+}
+
+#[tauri::command]
+pub fn secrets_get(key: String) -> Result<Option<String>, String> {
+    SecretsManager::get_secret(&key)
+}
+
+#[tauri::command]
+pub fn secrets_delete(key: String) -> Result<(), String> {
+    SecretsManager::delete_secret(&key)
+}
+
+#[tauri::command]
+pub fn secrets_list() -> Result<Vec<String>, String> {
+    SecretsManager::list_keys()
+}
+
+#[tauri::command]
+pub fn secrets_has_keyring() -> bool {
+    SecretsManager::has_keyring()
+}
+
+#[tauri::command]
+pub fn secrets_known_keys() -> Vec<String> {
+    crate::secrets::KNOWN_KEYS
+        .iter()
+        .map(|s| s.to_string())
+        .collect()
+}