feat(memory): config embedders (LOT C2) + suggestion contextuelle (LOT C3) + contexte projet partagé

- LOT C2 (§14.5.3) : use cases de configuration des embedders déclaratifs
  (List/Save/Delete + DescribeEmbedderEngines : modèles ONNX recommandés,
  environnement local détecté, stratégies compilées). UI EmbedderSettings.
- LOT C3 (§14.5.5) : suggestion contextuelle best-effort à l'activation quand la
  mémoire dépasse le budget de recall sans embedder configuré (event
  EmbedderSuggested, anti-spam 1×/session, « ne plus demander »).
- Contexte projet partagé .ideai/CONTEXT.md (model-agnostic) injecté à tous les
  agents/profils au lancement, avant la persona. UI ProjectContextPanel.

Tests : backend workspace vert (0 échec) ; frontend 306/306.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/features/embedder/useEmbedder.ts

@@ -0,0 +1,118 @@
+/**
+ * `useEmbedder` — view-model hook for the memory/embedder settings (L14 / C2).
+ *
+ * Owns the configured embedder profiles and the available engines (with the
+ * build-time feature flags). Consumes {@link EmbedderGateway} exclusively; never
+ * touches `invoke()` or `@tauri-apps/api`, keeping the component layer testable
+ * with mock gateways (ARCHITECTURE §1.3).
+ *
+ * Note: the embedder change takes effect at the *next app start* — this hook
+ * just persists the choice; it does not hot-swap the live recall tier.
+ */
+
+import { useCallback, useEffect, useState } from "react";
+
+import type { EmbedderEngines, EmbedderProfile, GatewayError } from "@/domain";
+import { useGateways } from "@/app/di";
+
+/** What the embedder settings UI needs from this hook. */
+export interface EmbedderViewModel {
+  /** The configured embedder profiles. */
+  profiles: EmbedderProfile[];
+  /** The available engines + build feature flags, or `null` until loaded. */
+  engines: EmbedderEngines | null;
+  /** The single active profile (first configured), or `null` (naïve/None). */
+  active: EmbedderProfile | null;
+  /** Last error message, or `null`. */
+  error: string | null;
+  /** Whether a request is in flight. */
+  busy: boolean;
+  /** Reloads profiles + engines. */
+  refresh: () => Promise<void>;
+  /** Persists a profile (create/replace by id) and refreshes. */
+  saveProfile: (profile: EmbedderProfile) => Promise<void>;
+  /** Deletes a profile by id and refreshes. */
+  deleteProfile: (embedderId: string) => Promise<void>;
+}
+
+function describe(e: unknown): string {
+  if (e && typeof e === "object" && "message" in e) {
+    return String((e as GatewayError).message);
+  }
+  return String(e);
+}
+
+export function useEmbedder(): EmbedderViewModel {
+  const { embedder } = useGateways();
+
+  const [profiles, setProfiles] = useState<EmbedderProfile[]>([]);
+  const [engines, setEngines] = useState<EmbedderEngines | null>(null);
+  const [error, setError] = useState<string | null>(null);
+  const [busy, setBusy] = useState(false);
+
+  const refresh = useCallback(async () => {
+    setBusy(true);
+    setError(null);
+    try {
+      const [list, eng] = await Promise.all([
+        embedder.listEmbedderProfiles(),
+        embedder.describeEmbedderEngines(),
+      ]);
+      setProfiles(list);
+      setEngines(eng);
+    } catch (e) {
+      setError(describe(e));
+    } finally {
+      setBusy(false);
+    }
+  }, [embedder]);
+
+  useEffect(() => {
+    void refresh();
+  }, [refresh]);
+
+  const saveProfile = useCallback(
+    async (profile: EmbedderProfile) => {
+      setBusy(true);
+      setError(null);
+      try {
+        await embedder.saveEmbedderProfile(profile);
+        await refresh();
+      } catch (e) {
+        setError(describe(e));
+        setBusy(false);
+      }
+    },
+    [embedder, refresh],
+  );
+
+  const deleteProfile = useCallback(
+    async (embedderId: string) => {
+      setBusy(true);
+      setError(null);
+      try {
+        await embedder.deleteEmbedderProfile(embedderId);
+        await refresh();
+      } catch (e) {
+        setError(describe(e));
+        setBusy(false);
+      }
+    },
+    [embedder, refresh],
+  );
+
+  // The active embedder is the first configured profile (the backend keeps a
+  // single active embedder); `null` ⇒ naïve recall (None).
+  const active = profiles.find((p) => p.strategy !== "none") ?? null;
+
+  return {
+    profiles,
+    engines,
+    active,
+    error,
+    busy,
+    refresh,
+    saveProfile,
+    deleteProfile,
+  };
+}