feat(frontend): viewer humain fil-par-paire en lecture seule (LS7)

Frontend pur, lecture seule, consomme read_conversation_page (LS6) ; aucun changement backend.
- adapters : conversation.ts (gateway) + conversationNormalization.ts.
- features/conversations : useConversationThread, ConversationViewer, index.
- domain (types LS7), ports (port + Gateways.conversation), adapters/index, mock (+ clé inventaire).
- features/workstate : ProjectWorkStatePanel prop onOpenConversation.
- features/projects : ProjectsView swap viewer.
- tests (QA, verts) : conversationNormalization, mock/conversationGateway,
  useConversationThread, ConversationViewer, ProjectsView.ls7. tsc 0, vitest 443/443 (+36).

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

frontend/src/features/projects/ProjectsView.tsx

@@ -39,6 +39,7 @@ import { MemoryPanel } from "@/features/memory";
 import { EmbedderSettings } from "@/features/embedder";
 import { PermissionsPanel } from "@/features/permissions";
 import { ProjectWorkStatePanel } from "@/features/workstate";
+import { ConversationViewer } from "@/features/conversations";
 import { GitPanel, GitGraphView } from "@/features/git";
 import { Button, Input, Panel, Tabs, cn } from "@/shared";
 import { useGateways } from "@/app/di";
@@ -79,6 +80,12 @@ export function ProjectsView() {
   // truth. `kind` decides whether the main area is the terminal grid or the git
   // graph view.
   const [activeLayout, setActiveLayout] = useState<LayoutInfo | null>(null);
+  // When set, the main area swaps the terminal grid for the read-only
+  // conversation viewer (LS7) — pure local UI state, same mechanic as the
+  // terminal↔gitGraph swap; **not** a backend layout kind.
+  const [viewerConversationId, setViewerConversationId] = useState<
+    string | null
+  >(null);
 
   const active = vm.openTabs.find((t) => t.id === vm.activeTabId) ?? null;
 
@@ -89,6 +96,7 @@ export function ProjectsView() {
   // loading it against the new project's store fails with "not found: layout X".
   useEffect(() => {
     setActiveLayout(null);
+    setViewerConversationId(null);
   }, [active?.id]);
 
   const activeLayoutKind = activeLayout?.kind ?? "terminal";
@@ -287,7 +295,10 @@ export function ProjectsView() {
 
             {/* Work-state panel */}
             {sidebarTab === "work" && active && (
-              <ProjectWorkStatePanel projectId={active.id} />
+              <ProjectWorkStatePanel
+                projectId={active.id}
+                onOpenConversation={setViewerConversationId}
+              />
             )}
             {sidebarTab === "work" && !active && (
               <p className="text-sm text-muted">Open a project to view work state.</p>
@@ -348,7 +359,14 @@ export function ProjectsView() {
 
         {/* ── Main: terminal grid or git graph (fills remaining height) ── */}
         <main className="flex flex-1 flex-col overflow-hidden">
-          {active ? (
+          {active && viewerConversationId ? (
+            <ConversationViewer
+              key={`${active.id}-${viewerConversationId}`}
+              projectId={active.id}
+              conversationId={viewerConversationId}
+              onClose={() => setViewerConversationId(null)}
+            />
+          ) : active ? (
             <>
               <LayoutTabs
                 projectId={active.id}