Add project scaffolding and build configuration

Initialize the session-viewer project — a tool for browsing, filtering,
redacting, and exporting Claude Code session logs as self-contained HTML.

Scaffolding includes:
- package.json with Express server + React client dual-stack setup,
  dev/build/test/lint/typecheck scripts, and a CLI bin entry point
- TypeScript configs: base tsconfig.json (ESNext, bundler resolution,
  strict, react-jsx) and tsconfig.server.json extending it for the
  Express server compilation target
- Vite config: React plugin, Tailscale-aware dev server on :3847 with
  API proxy to :3848, client build output to dist/client
- Vitest config: node environment, test discovery from tests/unit and
  src/client/components
- ESLint flat config: typescript-eslint recommended, unused-vars with
  underscore exception
- Tailwind CSS config scoped to src/client, PostCSS with autoprefixer
- Playwright config: Chromium-only E2E against dev server
- bin/session-viewer.js: CLI entry point that re-execs via tsx with
  browser-open flag

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

vite.config.ts

@@ -0,0 +1,30 @@
+import { defineConfig } from "vite";
+import react from "@vitejs/plugin-react";
+import path from "path";
+
+export default defineConfig({
+  plugins: [react()],
+  root: "src/client",
+  resolve: {
+    alias: {
+      "@shared": path.resolve(__dirname, "src/shared"),
+    },
+  },
+  server: {
+    port: 3847,
+    // Vite only supports one host. Use Tailscale IP so it's reachable
+    // from both the local machine (via the TS IP) and the tailnet.
+    // localhost:3847 won't work for the Vite dev server — use the TS IP.
+    host: "100.84.4.113",
+    proxy: {
+      "/api": {
+        target: "http://127.0.0.1:3848",
+        changeOrigin: true,
+      },
+    },
+  },
+  build: {
+    outDir: "../../dist/client",
+    emptyOutDir: true,
+  },
+});