feat: add visual review workflow for automated PR screenshot comparison

Adds a label-triggered GitHub Actions workflow that automatically generates
before/after screenshots when web UI changes are made in a PR. Uses smart
dependency analysis to only screenshot pages affected by the changed files.

Key components:
- Reverse dependency analyzer: traces changed files through the import graph
  to find which +page.svelte routes are affected
- Screenshot scenarios: Playwright tests using existing mock-network infrastructure
  (no Docker/backend needed) for fast, deterministic screenshots
- Pixel comparison: generates diff images highlighting changed pixels
- GitHub Actions workflow: triggered by 'visual-review' label, posts results
  as a PR comment with change percentages per page

https://claude.ai/code/session_01XSTqDJXuR4jaLN7SGm3uES

e2e/playwright.screenshot.config.ts

@@ -0,0 +1,27 @@
+import { defineConfig, devices } from '@playwright/test';
+
+const baseUrl = process.env.SCREENSHOT_BASE_URL ?? 'http://127.0.0.1:4173';
+
+export default defineConfig({
+  testDir: './src/screenshots',
+  testMatch: /run-scenarios\.ts/,
+  fullyParallel: false,
+  forbidOnly: !!process.env.CI,
+  retries: 0,
+  reporter: 'list',
+  use: {
+    baseURL: baseUrl,
+    screenshot: 'off',
+    trace: 'off',
+  },
+  workers: 1,
+  projects: [
+    {
+      name: 'screenshots',
+      use: {
+        ...devices['Desktop Chrome'],
+        viewport: { width: 1920, height: 1080 },
+      },
+    },
+  ],
+});