ODIAN Launcher: hub, installer, local stack and remembered sign-in (M2)

- Tauri 2 + Svelte 5 hub in ODIAN dark: patch notes and roadmap, install and
  update progress, local stack status, settings with DHP and MCP addresses.
- crates/odian-packs: Rust installer for pack format v1 (signed manifests,
  resumable verified downloads from a DHP or an ODIAN drive), byte-compatible
  with ODIAN_MCP_Server's Python odian_packs.
- Local stack on 127.0.0.1: Postgres -> Engine API -> tile server -> Odian.exe,
  one Job Object, clean shutdown; database snapshot restore on a fresh PC only.
- Keycloak sign-in through the DHP's MCP, remembered until Sign out with a
  launcher session that renews the Engine token silently.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/lib/backend.ts

@@ -0,0 +1,65 @@
+// The ONLY module that touches Tauri. Everything else talks to `backend`.
+// Inside the Tauri webview this is a thin typed wrapper over invoke/listen
+// (docs/CONTRACT.md); in a plain browser it is the timer-driven mock.
+import { invoke } from "@tauri-apps/api/core";
+import { listen } from "@tauri-apps/api/event";
+import { getCurrentWindow } from "@tauri-apps/api/window";
+import { createMockBackend } from "./mock";
+import type {
+  Backend,
+  InstallProgress,
+  LogEvent,
+  Settings,
+  SignInResult,
+  StackEvent,
+  Status,
+  Unlisten,
+} from "./types";
+
+export const IN_TAURI = typeof window !== "undefined" && "__TAURI_INTERNALS__" in window;
+
+function subscribe<T>(event: string, handler: (payload: T) => void): Promise<Unlisten> {
+  return listen<T>(event, (e) => handler(e.payload));
+}
+
+function createTauriBackend(): Backend {
+  const win = getCurrentWindow();
+
+  return {
+    kind: "tauri",
+    mock: null,
+    window: {
+      minimize: () => win.minimize(),
+      toggleMaximize: () => win.toggleMaximize(),
+      close: () => win.close(),
+      isMaximized: () => win.isMaximized(),
+      onResized: (handler) => win.onResized(() => handler()),
+    },
+
+    getStatus: () => invoke<Status>("get_status"),
+    refresh: () => invoke<Status>("refresh"),
+    signIn: () => invoke<SignInResult>("sign_in"),
+    cancelSignIn: () => invoke<void>("cancel_sign_in"),
+    signOut: () => invoke<Status>("sign_out"),
+    getNotes: (version) => invoke<string>("get_notes", version === undefined ? {} : { version }),
+    openExternal: (url) => invoke<void>("open_external", { url }),
+    getStack: () => invoke<StackEvent[]>("get_stack"),
+    startInstall: () => invoke<void>("start_install"),
+    startInstallFromDrive: (path) => invoke<void>("start_install_from_drive", { path }),
+    cancelInstall: () => invoke<void>("cancel_install"),
+    launch: () => invoke<void>("launch"),
+    stop: () => invoke<void>("stop"),
+    getSettings: () => invoke<Settings>("get_settings"),
+    setSettings: (settings) => invoke<Status>("set_settings", { settings }),
+    pickFolder: () => invoke<string | null>("pick_folder"),
+
+    onStatus: (handler) => subscribe<Status>("status", handler),
+    onInstallProgress: (handler) => subscribe<InstallProgress>("install-progress", handler),
+    onStack: (handler) => subscribe<StackEvent>("stack", handler),
+    onLog: (handler) => subscribe<LogEvent>("log", handler),
+  };
+}
+
+export const backend: Backend = IN_TAURI
+  ? createTauriBackend()
+  : createMockBackend(new URLSearchParams(window.location.search));