Fix GFM markdown rendering in production

kramdown-parser-gfm was only pulled in transitively by solargraph, which
lives in the development group. Production installs therefore lacked the
GFM parser, so rendering any rich-text content page raised
"Kramdown::Error: kramdown has no parser to handle the specified input
format: GFM".

Declare kramdown-parser-gfm as a direct dependency and add a regression
spec for ApplicationHelper#markdown_to_html.

Fixes #264

Gemfile

@@ -43,6 +43,7 @@ gem 'flipper-ui'
 gem 'gpgme', '~> 2.0.24'
 gem 'zbase32', '~> 0.1.1'
 gem 'kramdown'
+gem 'kramdown-parser-gfm'
 gem 'device_detector'
 
 # HTTP requests

Gemfile.lock

@@ -561,6 +561,7 @@ DEPENDENCIES
   importmap-rails
   jbuilder (~> 2.15)
   kramdown
+  kramdown-parser-gfm
   letter_opener
   letter_opener_web
   listen (~> 3.2)

spec/helpers/application_helper_spec.rb

@@ -1,4 +1,13 @@
 require 'rails_helper'
 
 describe ApplicationHelper do
+  describe "#markdown_to_html" do
+    it "renders GFM input" do
+      expect(helper.markdown_to_html("**bold**")).to include("<strong>bold</strong>")
+    end
+
+    it "renders GFM-specific syntax" do
+      expect(helper.markdown_to_html("~~struck~~")).to include("<del>struck</del>")
+    end
+  end
 end