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>

eslint.config.js

@@ -0,0 +1,18 @@
+import js from "@eslint/js";
+import tseslint from "typescript-eslint";
+
+export default tseslint.config(
+  js.configs.recommended,
+  ...tseslint.configs.recommended,
+  {
+    ignores: ["dist/**", "node_modules/**", "bin/**"],
+  },
+  {
+    rules: {
+      "@typescript-eslint/no-unused-vars": [
+        "error",
+        { argsIgnorePattern: "^_" },
+      ],
+    },
+  }
+);