refactor: centralize types and extract seed data (Task 1.1)

- Create types/ directory with centralized domain types:
  session, tool-instance, tool-type, git-repository, config-folder,
  tool-config, project, user, api-response
- Remove inline type definitions from API modules;
  re-export from types/ for backward compatibility
- Update state/sessions.tsx to import Session from types/session.ts
- Update all consumer components/pages to import from types/
- Extract seed_builtin_tool_types from main.py to
  seeds/builtin_tool_types.py
- Create types/index.ts barrel export

Quality gates: tsc (pass), eslint (pass), Python syntax (pass)

apps/web/src/api/sessions.ts

@@ -1,31 +1,9 @@
 import { apiClient } from "./client";
+import type { Session } from "../types/session";
+import type { ToolInstance } from "../types/tool-instance";
 
-export interface ToolInstance {
-  id: string;
-  name: string;
-  display_name: string;
-  tool_type_id: string;
-  tool_type_name: string;
-  tool_type_interfaces: string[];
-  status: string;
-  url: string | null;
-  port: number | null;
-  created_at: string;
-}
-
-export interface Session {
-  id: string;
-  display_name: string;
-  tool_type_name: string;
-  tool_icon: string;
-  tool_type_interfaces: string[];
-  repository_name: string;
-  repository_id: string;
-  project_name: string;
-  project_id: string;
-  status: string;
-  url: string | null;
-}
+export type { Session } from "../types/session";
+export type { ToolInstance } from "../types/tool-instance";
 
 export async function listInstances(
   projectId: string,