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/adapters/mock/mock.test.ts

@@ -15,6 +15,7 @@ describe("createMockGateways", () => {
   it("exposes all gateways", () => {
     expect(Object.keys(gateways).sort()).toEqual([
       "agent",
+      "conversation",
       "embedder",
       "git",
       "input",