feat: persist LLM/TTS backend choice + reset/repeat context + quick-search + diagnostics

Closes the UX hole I created in 5c72450: voice-swap to Ollama used to vanish
after restart. Plus P0.3 (long-standing roadmap item) and two new packs.

Persistent backend choice (crates/jarvis-core/src/db/structs.rs)
  - Settings struct gains llm_backend + tts_backend fields (both String,
    "" / "auto" = follow env/auto-detect).
  - set("llm_backend", "groq"|"ollama"|"auto") validates input.
  - llm::init_global() reads DB first, then JARVIS_LLM env, then auto-detect.
  - llm::swap_to() now persists the choice via db::save_settings.
  - Voice swap "переключись на локальный" now survives restart.

Shared conversation history (P0.3, crates/jarvis-core/src/llm/mod.rs)
  - HISTORY: Lazy<RwLock<Option<ConversationHistory>>> singleton.
  - Helpers: init_history, history_push_user, history_push_assistant,
    history_snapshot, history_clear, history_pop_last_user, history_last_assistant.
  - llm_fallback migrated off its own Mutex<History> — now reads/writes shared.
  - ConversationHistory gains last_assistant() method.

New Lua APIs
  - jarvis.llm_reset()        → clear conversation turns (keeps system prompt).
  - jarvis.llm_last_reply()   → string or nil (last assistant message text).
  - jarvis.health()           → debug table {tts_backend, llm_backend, llm_model,
                                  active_profile, memory_facts, scheduled_tasks,
                                  language, voice, microphone, vosk_model,
                                  noise_suppression}. No secrets included.

New voice packs
  - resources/commands/llm_context/   (P0.3)
    * "сбрось контекст" / "забудь разговор"     → llm.reset
    * "повтори последнее" / "повтори ответ"     → llm.repeat (uses last_assistant)
  - resources/commands/quick_search/   (imba P1 item)
    * "найди в гугле <X>" / "загугли <X>"
    * Uses DuckDuckGo Instant Answer API (api.duckduckgo.com, no key required).
      Pulls AbstractText or RelatedTopics into LLM prompt; falls back to pure
      LLM knowledge if DDG returns nothing useful. Speaks 2-4 sentence answer.
  - resources/commands/diagnostics/
    * "диагностика" / "доложи о себе" / "статус"
    * Reads jarvis.health() and speaks a one-line summary. Useful when
      debugging — user can read out their current state for a bug report.

Build: cargo build --release -p jarvis-app -p jarvis-gui green.
Tests: 52/52 jarvis-core unit tests pass.

crates/jarvis-core/src/db/structs.rs

@@ -33,6 +33,19 @@ pub struct Settings {
     pub language: String,
 
     pub api_keys: ApiKeys,
+
+    /// Preferred LLM backend ("groq" | "ollama" | empty=auto). Empty/auto means
+    /// the JARVIS_LLM env var or auto-detect (Groq if GROQ_TOKEN set else Ollama)
+    /// decides. When the user runs `jarvis.llm_switch(...)` the result is
+    /// persisted here so it survives restart.
+    #[serde(default)]
+    pub llm_backend: String,
+
+    /// Preferred TTS backend ("sapi" | "piper" | "silero" | empty=auto). Same
+    /// rules as `llm_backend`. Empty falls back to JARVIS_TTS env var, then
+    /// auto-detect.
+    #[serde(default)]
+    pub tts_backend: String,
 }
 
 fn default_intent_backend() -> String { config::DEFAULT_INTENT_BACKEND.to_string() }
@@ -60,6 +73,8 @@ impl Settings {
             "language"                  => Some(self.language.clone()),
             "api_key__picovoice"        => Some(self.api_keys.picovoice.clone()),
             "api_key__openai"           => Some(self.api_keys.openai.clone()),
+            "llm_backend"               => Some(self.llm_backend.clone()),
+            "tts_backend"               => Some(self.tts_backend.clone()),
             _ => None,
         }
     }
@@ -120,6 +135,22 @@ impl Settings {
             "api_key__openai" => {
                 self.api_keys.openai = val.to_string();
             }
+            "llm_backend" => {
+                // empty / "auto" / "groq" / "ollama" — anything else rejected
+                match val.trim().to_lowercase().as_str() {
+                    "" | "auto" => self.llm_backend = String::new(),
+                    "groq" => self.llm_backend = "groq".into(),
+                    "ollama" => self.llm_backend = "ollama".into(),
+                    other => return Err(format!("unknown llm_backend: '{}'", other)),
+                }
+            }
+            "tts_backend" => {
+                match val.trim().to_lowercase().as_str() {
+                    "" | "auto" => self.tts_backend = String::new(),
+                    "sapi" | "piper" | "silero" => self.tts_backend = val.to_lowercase(),
+                    other => return Err(format!("unknown tts_backend: '{}'", other)),
+                }
+            }
             _ => return Err(format!("unknown setting: '{}'", key)),
         }
         Ok(())
@@ -142,6 +173,8 @@ impl Settings {
             "language",
             "api_key__picovoice",
             "api_key__openai",
+            "llm_backend",
+            "tts_backend",
         ]
     }
 }
@@ -173,6 +206,9 @@ impl Default for Settings {
                 picovoice: String::from(""),
                 openai: String::from(""),
             },
+
+            llm_backend: String::new(),
+            tts_backend: String::new(),
         }
     }
 }