feat(ai): sidebar epuree, suivi visuel des requetes et correctif menu @ (BUG-012, #82)

BUG-012: le menu @ est toujours rendu et _mentionVault() retombe sur le vault de contexte puis le premier vault disponible. #82: suppression des intitules Fournisseur & modele / Fournisseur:, description du contexte General en info-bulle, placeholder retire, bouton Envoyer en emoji, indicateur d'activite (envoi/reception/outils/confirmation/succes/echec), SW_VERSION v5.

tests/frontend/ai.test.mjs

@@ -1024,6 +1024,52 @@ async function main() {
     localStorage.clear();
   });
 
+  // ── 42b. Assistant activity indicator (#82) ──
+  await test("activity indicator reflects the request workflow", () => {
+    const b = new BooksLM();
+    b._panel = b._render();
+    document.body.appendChild(b._panel);
+    const el = b._panel.querySelector(".bookslm-activity");
+    assert.ok(el, "activity element present");
+    assert.ok(el.classList.contains("hidden"), "hidden initially");
+    b._setActivity("working", "Envoi…");
+    assert.ok(!el.classList.contains("hidden"), "shown while working");
+    assert.ok(el.classList.contains("working"));
+    assert.equal(el.querySelector(".bookslm-activity-text").textContent, "Envoi…");
+    b._setActivity("error", "Échec");
+    assert.ok(el.classList.contains("error"));
+    b._setActivity("idle");
+    assert.ok(el.classList.contains("hidden"), "hidden when idle");
+    b._panel.remove();
+  });
+
+  await test("mention menu falls back to an available vault", async () => {
+    localStorage.clear();
+    const stateMod = await import(pathToFileURL(path.join(JS_DIR, "state.js")).href);
+    const prev = stateMod.state.allVaults;
+    stateMod.state.allVaults = [{ name: "FallbackVault" }];
+    let lastUrl = "";
+    globalThis.fetch = async (url) => {
+      lastUrl = String(url);
+      if (lastUrl.includes("/files")) {
+        return { ok: true, status: 200, json: async () => ({ files: [{ path: "a.md" }] }) };
+      }
+      return { ok: true, status: 200, json: async () => ({}) };
+    };
+    const b = new BooksLM();
+    b._panel = b._render();
+    document.body.appendChild(b._panel);
+    b._mode = MODE.GENERAL;
+    b._vault = null;
+    await b._showMentionMenu("");
+    const menu = b._panel.querySelector(".bookslm-mention-menu");
+    assert.ok(!menu.classList.contains("hidden"), "empty-query menu is shown");
+    assert.ok(lastUrl.includes("FallbackVault"), "uses the fallback vault");
+    stateMod.state.allVaults = prev;
+    b._panel.remove();
+    localStorage.clear();
+  });
+
   // ── Summary ──
   console.log(`\n${passCount}/${testCount} tests passed`);
   if (passCount !== testCount) {