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_de.arb

@@ -1806,7 +1806,7 @@
   "examPacePages": "{count, plural, =1{1 Seite am Tag} other{{count} Seiten am Tag}}",
   "examPaceDone": "Alles Neue ist gelernt: bis zur Prüfung wiederholen.",
   "examStudy": "Für die Prüfung lernen",
-  "examSheetHelp": "Wähle, was die Prüfung abdeckt und wann sie ist. Alles Neue wird dann so verteilt, dass es eine Woche vorher gelernt ist und die letzte Woche zum Wiederholen bleibt. Das Datum siehst nur du.",
+  "examSheetHelp": "Wähle, was die Prüfung abdeckt und wann sie ist. Alles Neue wird dann so verteilt, dass es eine Woche vor der Prüfung gelernt ist, oder auf halbem Weg dorthin, wenn sie näher ist, und der Rest zum Wiederholen bleibt. Das Datum siehst nur du.",
   "examDate": "Prüfungsdatum",
   "examSheetPlans": "Prüfungspläne dieses Lernsets",
   "examSheetFromTags": "Aus den Stufen-Tags",