feat(ui): a question mark that keeps a section's explanation one tap away

HelpButton, beside a section's or a setting's title: the explanation opens
in a small dialog, under its title where given, rather than standing in the
everyday view (the developer's review of 3 Nov 2026). FieldInfoButton's
tooltip stays for a field's short hint.

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

packages/halkyon_ui/lib/l10n/halkyon_ui_en.arb

@@ -1,6 +1,5 @@
 {
   "@@locale": "en",
-
   "brand": "Halkyon",
   "@brand": {
     "description": "The brand's word, above an app's own in its wordmark (\"Halkyon\" over \"Learn\")."
@@ -38,5 +37,9 @@
         "type": "String"
       }
     }
+  },
+  "explain": "What's this?",
+  "@explain": {
+    "description": "Tooltip of the question mark that opens a text explaining a section or setting (HelpButton)."
   }
 }