chore: Halkyon Learn moves into apps/learn

Its server, client, app and curator, and the tools that write its
assets (maps, Anki fixtures, icons), as they are; paths are fixed in
the commits that follow.

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

apps/learn/halkyon_learn_flutter/lib/l10n/app_en_GB.arb

@@ -0,0 +1,3 @@
+{
+  "@@locale": "en_GB"
+}

apps/learn/halkyon_learn_flutter/lib/l10n/app_en_US.arb

@@ -0,0 +1,26 @@
+{
+  "@@locale": "en_US",
+
+  "studysetSheetColor": "Color",
+  "tagSheetColor": "Color",
+  "formatAlignCenter": "Center",
+  "homeCustomise": "Customize",
+  "studysetSheetLicense": "License",
+  "studysetSheetLicenseLabel": "License",
+  "studysetSheetLicenseHolder": "License holder",
+  "studysetSheetLicenseHolderHelper": "Who to credit: a name or an organization.",
+  "studysetSheetLicenseNewer": "{license} is an older Creative Commons license. Its current version, {current}, may fit better.",
+  "licenseChooserTitle": "Choose a license",
+  "licenseChooserSubtitle": "A few questions to find the Creative Commons license that fits how you want your work shared.",
+  "licenseChooserIntroYes": "Yes, share it under a license",
+  "licenseChooserAdaptationsShareAlikeDetail": "Adaptations carry this same license (SA).",
+  "licenseChooserRecommended": "Recommended license",
+  "licenseChooserUse": "Use this license",
+  "licenseSummaryBySa": "Others may share and adapt it, even commercially, if they credit you and license their adaptations the same way.",
+  "licenseSummaryByNcSa": "Others may share and adapt it non-commercially, with credit and the same license on adaptations.",
+  "licenseSummaryByNcNd": "Others may share it non-commercially, with credit, but may not adapt it: the strictest Creative Commons license.",
+  "publishLicence": "License",
+  "publishLicenceNotOpen": "These studysets' licenses don't allow passing them on: {titles}",
+  "exploreReadLicence": "Read the license",
+  "suggestNoDerivatives": "Its license forbids adaptations: no suggestions."
+}