feat: Phase 4-5 — demo mode, polish, deploy, and bug fixes

Add demo mode with mock data provider, Docker deployment, Playwright
tests, PostHog analytics, error boundaries, and SEO metadata. Fix
residences API response unwrapping, kanban drag-and-drop with optimistic
updates, trailing slash proxy redirects, and column name mismatches with
Go API.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

playwright.config.ts

@@ -0,0 +1,34 @@
+import { defineConfig, devices } from "@playwright/test";
+
+export default defineConfig({
+  testDir: "./tests",
+  fullyParallel: true,
+  forbidOnly: !!process.env.CI,
+  retries: process.env.CI ? 2 : 0,
+  workers: process.env.CI ? 1 : undefined,
+  reporter: "html",
+  use: {
+    baseURL: "http://localhost:3000",
+    screenshot: "only-on-failure",
+    trace: "on-first-retry",
+  },
+  projects: [
+    {
+      name: "Desktop Chrome",
+      use: { ...devices["Desktop Chrome"] },
+    },
+    {
+      name: "Mobile Safari",
+      use: { ...devices["iPhone 14"] },
+    },
+    {
+      name: "Tablet",
+      use: { viewport: { width: 768, height: 1024 } },
+    },
+  ],
+  webServer: {
+    command: "npm run dev",
+    url: "http://localhost:3000",
+    reuseExistingServer: !process.env.CI,
+  },
+});