feat(ui): required fields say so with a stronger outline and "Required"

Only a studyset's title was marked, with an asterisk; the other fields
nothing can be saved without said nothing until Save stayed grey. Every
required field now has, while empty, its outline (or its line) stronger
in the primary colour and "Required" below it, before its own helper
text: a studyset's and a path's title (in the sheet and the editor, in
place of its error), a tag's name, a PDF's new name, a YouTube video's
link, a link's address, the handle, an instance's address and a custom
search engine's address. Once written, an empty line keeps its place,
so nothing below moves.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMm7usAo6q9ZZRoPkWjoxi
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/lib/l10n/app_en.arb

@@ -154,7 +154,6 @@
   "pathCreateTitle": "New path",
   "pathCreate": "Create",
   "pathTitleLabel": "Title",
-  "pathTitleRequired": "A path needs a title.",
   "pathSecondaryTitleLabel": "Secondary title",
   "pathSecondaryTitleHint": "E.g. the title in the language you learn",
   "pathEditTitle": "Edit path",
@@ -465,6 +464,7 @@
   "languageChineseSimplified": "Chinese (Simplified)",
   "languageChineseTraditional": "Chinese (Traditional)",
   "fieldRequired": "required",
+  "fieldRequiredHelper": "Required",
 
   "exploreTitle": "Explore",
   "exploreEmpty": "Studysets that others share will appear here.",