fix(studyset): an exam's pace keeps its rate, and plans hold what they show

The pace kept a week for reviews only while more than two weeks were
left: followed from further off, it halved its rate at two weeks and
brought new cards until the exam. It now keeps a week, or half the days
left when the exam is closer, and the sheet says so.

An exam whose plan is gone said "Today", as the days left came from the
pace, which needs the plan; they are now counted from the date alone.
Once the exam's day was over, the study plan still asked for everything
new at once and Study for the exam queued every new card; now neither
shows, and a session for it studies as usual.

Cards tagged HSK 3 alone offered HSK 1 and 2, plans covering nothing;
a level offers a plan only when it covers some tag present. The plan
editor kept topics deleted or moved out since and tags spelled in
another case, chosen but not shown, and ticking such a tag added it
twice; once it has read what a plan may cover, only that stays chosen,
each tag as the flashcards spell it.

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

@@ -2137,7 +2137,7 @@
   "@examPacePages": {"placeholders": {"count": {"type": "int"}}},
   "examPaceDone": "Everything new is learnt: review until the exam.",
   "examStudy": "Study for the exam",
-  "examSheetHelp": "Choose what the exam covers and when it is. Everything new is then spread so that it is learnt a week before, leaving the last week for reviews. Only you see the date.",
+  "examSheetHelp": "Choose what the exam covers and when it is. Everything new is then spread so that it is learnt a week before the exam, or halfway there when it is closer, leaving the rest for reviews. Only you see the date.",
   "examDate": "Exam date",
   "examSheetPlans": "This studyset’s exam plans",
   "examSheetFromTags": "From the level tags",