feat(gui): footer backend chips + Settings 'AI Backends' tab + i18n

Surfaces the LLM/TTS hot-swap UX in the GUI so the user doesn't need env vars
or voice commands. Tauri commands for this landed in b243e67; this commit
wires them to Svelte.

Footer (frontend/src/components/Footer.svelte)
  - Poll get_active_backends() every 5 s, show TTS / LLM / Profile chips.
  - Chips have border accents by kind (cyan = TTS, lime = LLM, orange = Profile).
  - Friendly title tooltips with full backend + model names.
  - Silent if jarvis-gui can't reach the command (e.g. cold start) — no UI flicker.

Settings — new "AI Backends" tab (frontend/src/routes/settings/index.svelte)
  - Status banner showing active LLM (with model name) + TTS + Profile.
  - LLM selector: Auto / Groq / Ollama → calls set_llm_backend on change
    (hot-swap, persisted to DB).
  - TTS selector: Auto / SAPI / Piper / Silero → calls set_tts_backend
    (persisted, effective on next jarvis-app restart).
  - "Reset context" button → llm_reset_context Tauri command.
  - Error toast for swap failures (e.g. Groq selected but no GROQ_TOKEN).
  - Tips alert: how to install Ollama / Piper / voice commands.
  - Loads prefs via db_read('llm_backend' / 'tts_backend') on mount.

i18n strings (ru/en/ua, .ftl files)
  - 14 new keys: settings-ai-backends, settings-ai-active, settings-ai-auto,
    settings-ai-applying, settings-ai-error, settings-ai-tips,
    settings-llm-backend{,-desc}, settings-tts-backend{,-desc},
    settings-llm-context{,-desc}, settings-llm-reset, settings-profile.

Build glue (crates/jarvis-gui/Cargo.toml)
  - jarvis-core dep now includes the `llm` feature so backends.rs can resolve
    `jarvis_core::llm::*` (was failing to compile after the global LLM module).

.gitignore
  - tools/piper/*.dll, piper.exe, voices/, espeak-ng-data/ — locally
    installed binaries from tools/piper/install.ps1, not committed.

Build: cargo build --release -p jarvis-gui green (4m, includes npm/Vite).
Test path: rebuild + launch jarvis-gui, /settings → "AI Backends" tab.

frontend/src/components/Footer.svelte

@@ -1,4 +1,7 @@
 <script lang="ts">
+    import { onMount, onDestroy } from "svelte"
+    import { invoke } from "@tauri-apps/api/core"
+
     import { appInfo, translations, translate } from "@/stores"
 
     $: t = (key: string) => translate($translations, key)
@@ -14,9 +17,72 @@
         upstreamLink = info.upstreamLink
         issuesLink = info.issuesLink
     })
+
+    // Active backends — refreshed every 5 seconds. Silently fails if the
+    // Tauri command isn't registered (e.g. running against an old jarvis-app).
+    interface ActiveBackends {
+        tts: string
+        llm: string
+        llm_model: string | null
+        profile: string
+    }
+
+    let backends: ActiveBackends | null = null
+    let pollId: ReturnType<typeof setInterval> | null = null
+
+    async function refreshBackends() {
+        try {
+            backends = await invoke<ActiveBackends>("get_active_backends")
+        } catch (err) {
+            // ignore
+        }
+    }
+
+    onMount(() => {
+        refreshBackends()
+        pollId = setInterval(refreshBackends, 5000)
+    })
+
+    onDestroy(() => {
+        if (pollId !== null) clearInterval(pollId)
+    })
+
+    function chipTitle(key: "tts" | "llm"): string {
+        if (!backends) return ""
+        if (key === "tts") return `TTS backend: ${backends.tts}`
+        const model = backends.llm_model ? ` (${backends.llm_model})` : ""
+        return `LLM backend: ${backends.llm}${model}`
+    }
+
+    function chipLabel(value: string): string {
+        const map: Record<string, string> = {
+            sapi: "SAPI",
+            piper: "Piper",
+            silero: "Silero",
+            groq: "Groq",
+            ollama: "Ollama",
+            none: "—",
+        }
+        return map[value] ?? value
+    }
 </script>
 
 <footer id="footer">
+    {#if backends}
+        <p class="backends" title="Активные движки. Меняются в /settings и голосом.">
+            <span class="chip chip-tts" title={chipTitle("tts")}>
+                <small>TTS</small> {chipLabel(backends.tts)}
+            </span>
+            <span class="chip chip-llm" title={chipTitle("llm")}>
+                <small>LLM</small> {chipLabel(backends.llm)}
+            </span>
+            {#if backends.profile && backends.profile !== "default"}
+                <span class="chip chip-profile" title="Active profile">
+                    <small>Profile</small> {backends.profile}
+                </span>
+            {/if}
+        </p>
+    {/if}
     <p>
         © {currentYear} J.A.R.V.I.S.
         <span class="edition" title="Rust + Tauri build">Rust</span>
@@ -62,6 +128,35 @@
             vertical-align: middle;
         }
 
+        .backends {
+            margin-bottom: 8px;
+
+            .chip {
+                display: inline-block;
+                margin: 0 3px;
+                padding: 2px 8px;
+                border-radius: 10px;
+                font-size: 11px;
+                font-weight: 500;
+                letter-spacing: 0.2px;
+                color: #cfd2d5;
+                background: rgba(255, 255, 255, 0.06);
+                border: 1px solid rgba(255, 255, 255, 0.08);
+
+                small {
+                    color: #6c6e71;
+                    margin-right: 3px;
+                    font-size: 9px;
+                    text-transform: uppercase;
+                    letter-spacing: 0.5px;
+                }
+
+                &.chip-tts { border-color: rgba(82, 254, 254, 0.25); }
+                &.chip-llm { border-color: rgba(138, 200, 50, 0.25); }
+                &.chip-profile { border-color: rgba(255, 168, 60, 0.25); }
+            }
+        }
+
         p {
             margin: 0;
             padding: 0;