Add specs to prevent flatpickr format string corruption in locale files

Finnish locale (openfoodfoundation#13360) had `flatpickr_date_format: "Vuosi"` (Finnish for
"Year") instead of `"d.m.Y"`. Translators on Transifex mistakenly translated
the format codes, producing strings like "V1749326700000o005 12:35" because
`u` is the flatpickr milliseconds token.

The format string fix was already applied in 95412bd (fixing fi.yml). This
commit adds regression tests to catch similar issues in any locale going
forward:

- Ruby spec validates all locale files have valid flatpickr format strings
  (only recognised token chars + separators), with a documented list of
  currently-known broken locales (cy, el, eu, ko, pa, sr, tr, uk).
- Jest test confirms `importFlatpickrLocale("fi")` returns a valid Finnish
  locale object with Finnish weekday and month names.

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

Author: gbathree

spec/javascripts/stimulus/flatpickr_controller_test.js

@@ -31,5 +31,15 @@ describe("FlatpickrController", () => {
       expect(locale).toHaveProperty("weekAbbreviation");
       expect(locale.weekAbbreviation).toBe("Sem");
     });
+
+    it("returns locale object for Finnish (fi)", async () => {
+      const controller = new FlatpickrController();
+      const locale = await controller.importFlatpickrLocale("fi");
+      expect(locale).toBeInstanceOf(Object);
+      expect(locale).toHaveProperty("weekdays");
+      expect(locale).toHaveProperty("months");
+      expect(locale.weekdays.shorthand).toContain("ma");
+      expect(locale.months.shorthand).toContain("tammi");
+    });
   });
 });

spec/lib/flatpickr_date_formats_spec.rb

@@ -0,0 +1,83 @@
+# frozen_string_literal: true
+
+require "yaml"
+
+# Valid flatpickr format token characters (single-character tokens and common separators).
+# See: https://flatpickr.js.org/formatting/
+#
+# Tokens: Z D F G H J K M S U W Y d h i j l m n s u w y
+# Separators: . - / : (space)
+# Escape: \  (makes the next character literal)
+VALID_FLATPICKR_FORMAT_REGEX = %r{\A[ZDFGHJKMSUWYdhijlmnsuwy.\-/: \\]+\z}
+
+# Known locales with incorrect (translator-provided) format strings.
+# These should be fixed in Transifex and removed from this list as they are corrected.
+# See: https://github.com/openfoodfoundation/openfoodnetwork/issues/13360
+KNOWN_BROKEN_FLATPICKR_LOCALES = %w[cy el eu ko pa sr tr uk].freeze
+
+RSpec.describe "Flatpickr date format strings in locale files" do
+  def locale_files
+    Rails.root.glob("config/locales/*.yml")
+  end
+
+  def date_picker_formats(file)
+    yaml = YAML.safe_load_file(file, permitted_classes: [Symbol])
+    return {} unless yaml
+
+    root = yaml.values.first
+    return {} unless root.is_a?(Hash)
+
+    date_picker = root.dig("spree", "date_picker")
+    return {} unless date_picker.is_a?(Hash)
+
+    {
+      date: date_picker["flatpickr_date_format"],
+      datetime: date_picker["flatpickr_datetime_format"]
+    }.compact
+  end
+
+  it "locale files are present" do
+    expect(locale_files).not_to be_empty
+  end
+
+  shared_examples "valid flatpickr format" do |format_key|
+    it "#{format_key} is a valid flatpickr format string in all locales" do
+      invalid = []
+
+      locale_files.each do |file|
+        locale_code = File.basename(file, ".yml")
+        next if KNOWN_BROKEN_FLATPICKR_LOCALES.include?(locale_code)
+
+        formats = date_picker_formats(file)
+        format = formats[format_key]
+        next unless format
+
+        invalid << "#{locale_code}: '#{format}'" unless format.match?(VALID_FLATPICKR_FORMAT_REGEX)
+      end
+
+      expect(invalid)
+        .to be_empty,
+            "Invalid #{format_key} found in locale files:\n" \
+            "#{invalid.join("\n")}\n\n" \
+            "Flatpickr format strings use single-character tokens (e.g. Y=year, m=month).\n" \
+            "They must not be translated. Valid tokens: ZDFGHJKMSUWYdhijlmnsuwy\n" \
+            "Valid separators: . - / : space\n" \
+            "See: https://flatpickr.js.org/formatting/"
+    end
+  end
+
+  include_examples "valid flatpickr format", :date
+  include_examples "valid flatpickr format", :datetime
+
+  describe "Finnish locale specifically" do
+    it "has valid flatpickr_date_format" do
+      formats = date_picker_formats(Rails.root.join("config/locales/fi.yml"))
+      expect(formats[:date]).to eq("d.m.Y")
+    end
+
+    it "has valid flatpickr_datetime_format" do
+      formats = date_picker_formats(Rails.root.join("config/locales/fi.yml"))
+      expect(formats[:datetime]).to eq("d.m.Y H:i")
+    end
+  end
+end