Capture screenshots and generate the diff viewer during the preview
workflow, then bake the output into the Docker image so it's served
at /snapshots/diff.html on the Fly preview app.

- Add /snapshots to Roda's static plugin (no-op in prod)
- Add --no-sandbox to Chrome options in CI (needed on Ubuntu)
- Install Ruby and run `rake snapshots:diff` before the Fly deploy

https://claude.ai/code/session_01Bgey3hrVy9av1SHrpdo5bU

diff --git a/.github/workflows/preview.yml b/.github/workflows/preview.yml
index 93981e5..68c3149 100644
--- a/.github/workflows/preview.yml
+++ b/.github/workflows/preview.yml
@@ -19,6 +19,22 @@ jobs:
       url: ${{ steps.deploy.outputs.url }}
     steps:
       - uses: actions/checkout@v6
+
+      - if: github.event.action != 'closed'
+        uses: ruby/setup-ruby@v1
+        with:
+          bundler-cache: true
+
+      - name: Capture snapshot diff
+        if: github.event.action != 'closed'
+        run: |
+          rake snapshots:diff
+          snapshot_dir="${XDG_CACHE_HOME:-$HOME/.cache}/ketchup/snapshots"
+          cp -r "$snapshot_dir" public/snapshots
+          cp public/favicon.svg public/snapshots/
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+
       - name: Deploy preview
         id: deploy
         uses: superfly/fly-pr-review-apps@1.5.0
diff --git a/lib/ketchup/snapshots.rb b/lib/ketchup/snapshots.rb
index 6b0aa82..b921cbf 100644
--- a/lib/ketchup/snapshots.rb
+++ b/lib/ketchup/snapshots.rb
@@ -96,10 +96,13 @@ module Ketchup
         FileUtils.rm_rf(@output_dir)
         @output_dir.mkpath
 
+        browser_options = { "force-device-scale-factor" => 2 }
+        browser_options["no-sandbox"] = nil if ENV["CI"]
+
         @browser = Ferrum::Browser.new(
           headless: true,
           window_size: VIEWPORTS.fetch("desktop"),
-          browser_options: { "force-device-scale-factor" => 2 }
+          browser_options: browser_options
         )
 
         @server.call(@browser) do |url|
diff --git a/lib/ketchup/web.rb b/lib/ketchup/web.rb
index 55c635d..9c05e15 100644
--- a/lib/ketchup/web.rb
+++ b/lib/ketchup/web.rb
@@ -11,7 +11,7 @@ require_relative "views/user/show"
 
 class Web < Roda
   plugin :halt
-  plugin :static, %w[ /css /js /favicon.svg ]
+  plugin :static, %w[ /css /js /favicon.svg /snapshots ]
   plugin :all_verbs
   plugin :sessions, secret: CONFIG.session_secret
   plugin :route_csrf, csrf_failure: :empty_403, check_request_methods: %w[POST]