feat(app): the app speaks British and American English and German

Settings offers English (UK), English (US) and Deutsch after the
device's language, each named in its own language. app_en.arb, the
template, is now British English (colour, centre) and the fallback for
any language the app doesn't speak; app_en_US.arb overrides the five
messages American English spells differently, and app_en_GB.arb is
empty, there so that Material's texts and dates are British too. The
German translation says "du" and keeps one set of terms throughout
(Lernset, Karteikarte, Lernpfad; Anki's own Stapel and Unterstapel in
the Anki import). arb_files_test checks that German has every message
of the template and that every translation keeps its placeholders.

resolveAppLocale picks the first language the device prefers that the
app speaks, in its country's variant where there is one; other English
is British, and a country alone never picks a language (Spanish in the
United States isn't American English), which Flutter's own resolution
would do.

Both MaterialApps take material_ui's GlobalMaterialLocalizations
delegates: gen-l10n's list brings flutter_localizations', which serve
only package:flutter/material.dart, so material_ui's widgets found no
Material texts in German and threw.

The editor's own text blocks show our placeholders while empty (list
item, to-do, quote, heading, and the desktop's / hint): the editor's
own follow a locale of their own, process-wide, in few languages. The
last texts written into widgets (the person's data failing to open,
the cards failing to read, the About dialog) moved into the arb files.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/lib/l10n/app_en.arb

@@ -35,14 +35,14 @@
   "studysetSheetTitleLabel": "Title",
   "studysetSheetSecondaryLabel": "Secondary title",
   "studysetSheetSecondaryHelper": "Shown as a small label above the title.",
-  "studysetSheetColor": "Color",
+  "studysetSheetColor": "Colour",
   "studysetSheetDiscardTitle": "Discard changes?",
   "studysetSheetDiscardMessage": "You have unsaved changes to this studyset. Discard them and close the form?",
   "studysetSheetDiscardConfirm": "Discard",
 
   "paletteColorName": "{name, select, carrot{Carrot} cerise{Cerise} cobalt{Cobalt} copper{Copper} enzian{Enzian} fern{Fern} forest{Forest} ocean{Ocean} olive{Olive} pacific{Pacific} pickle{Pickle} pine{Pine} pink{Pink} plum{Plum} purple{Purple} reef{Reef} sahara{Sahara} slateblue{Slate blue} soil{Soil} strawberry{Strawberry} other{{name}}}",
   "@paletteColorName": {"description": "A colour of the palette content can take, by its stored name.", "placeholders": {"name": {"type": "String"}}},
-  "colorPickerLabel": "Color: {color}",
+  "colorPickerLabel": "Colour: {color}",
   "@colorPickerLabel": {"placeholders": {"color": {"type": "String"}}},
 
   "studysetCountFlashcards": "{count, plural, =1{1 flashcard} other{{count} flashcards}}",
@@ -241,7 +241,7 @@
   "richTextSave": "Save",
   "studysetSheetDescription": "Description",
   "studysetSheetDescriptionAdd": "Add a description",
-  "studysetSheetInheritColor": "Same color as the studyset it is in",
+  "studysetSheetInheritColor": "Same colour as the studyset it is in",
 
   "exploreTitle": "Explore",
   "exploreEmpty": "Studysets that others share will appear here.",
@@ -267,6 +267,9 @@
   "settingsDeleteTitle": "Delete your account?",
   "settingsDeleteMessage": "Everything you have, your studysets, cards and reviews, is deleted on the server and on all your devices. This can't be undone.",
   "settingsAbout": "About Halkyon Learn",
+  "aboutLegalese": "Free software: the app is under the GNU Affero General Public License, version 3 or any later version, the server under the Server Side Public License, version 1.",
+  "aboutSourceCode": "The source of what this instance runs:",
+  "aboutSourceUnreachable": "Couldn't reach this instance to ask where its source is.",
   "settingsAppearance": "Appearance",
   "settingsAppearanceDevice": "Device theme",
   "settingsAppearanceLight": "Light",
@@ -308,7 +311,7 @@
   "@tagSheetNameExists": {"placeholders": {"name": {"type": "String"}}},
   "tagSheetNameMerges": "There is a tag '{name}' already: saving merges this one into it.",
   "@tagSheetNameMerges": {"placeholders": {"name": {"type": "String"}}},
-  "tagSheetColor": "Color",
+  "tagSheetColor": "Colour",
   "tagSheetCreate": "Create",
   "tagSheetSave": "Save",
   "tagManageTitle": "Manage tags",
@@ -342,6 +345,7 @@
   "cardFilterClear": "Clear all",
   "cardFilterApply": "Apply",
   "cardFilterNothing": "No flashcards have this tag.",
+  "cardListFailed": "Couldn't read the cards.",
   "cardListMore": "More",
   "cardListManageTags": "Manage tags",
 
@@ -402,7 +406,7 @@
   "composerConflict": "This flashcard was changed differently on two devices. Choose the version to keep of each part below.",
   "composerKeepThis": "Keep this",
   "formatAlignLeft": "Align left",
-  "formatAlignCenter": "Center",
+  "formatAlignCenter": "Centre",
   "formatAlignRight": "Align right",
   "formatBold": "Bold",
   "formatItalic": "Italic",
@@ -559,6 +563,7 @@
   "commonClose": "Close",
   "commonUndo": "Undo",
   "commonTryAgain": "Try again",
+  "personDataFailed": "Couldn't open this device's data.",
   "sessionUnreachable": "Couldn't reach the server.",
   "notFoundGone": "This no longer exists. It may have been deleted on another device.",
   "fileImage": "Image",
@@ -597,6 +602,9 @@
   "blockNumberedList": "Numbered list",
   "blockChecklist": "Checklist",
   "blockQuote": "Quote",
+  "blockListItem": "List item",
+  "blockChecklistItem": "To-do",
+  "editorSlashHint": "Type / to insert a block, or just start typing",
   "blockDivider": "Divider",
   "blockTable": "Table",
   "blockRemove": "Remove",