feat(app): wire LlmClient into voice loop with trigger-prefix fallback

new module crates/jarvis-app/src/llm_fallback.rs holds an optional
LlmClient + ConversationHistory built once at startup. if GROQ_TOKEN is
unset the module logs a warning and stays disabled — voice commands keep
working as before.

both the wake-word voice path (recognize_command in app.rs) and the
gui-side text command path (process_text_command) now check whether the
recognized phrase starts with one of the configured trigger phrases
(ru: 'скажи' / 'ответь' / 'произнеси'). when it does, the remainder of
the phrase is sent to Groq and the reply is published as a new
IpcEvent::LlmReply { text } so the gui can speak it.

on api error the trailing user turn is popped from history, the russian
fallback line is sent over ipc and a 'error' voice cue plays. the loop
itself never panics.

crates/jarvis-app/src/main.rs

@@ -18,6 +18,7 @@ mod log;
 
 // include app
 mod app;
+mod llm_fallback;
 
 // include tray
 // @TODO. macOS currently not supported for tray functionality.
@@ -55,6 +56,9 @@ fn main() -> Result<(), String> {
     // init i18n
     i18n::init(&settings.lock().language);
 
+    // init LLM fallback (no-op if GROQ_TOKEN missing)
+    llm_fallback::init();
+
     // init recorder
     if recorder::init().is_err() {
         app::close(1);