feat(study): limits switch on and off, step by 5, and take any number typed

Your calls (2026-10-11): a switch per limit, and steps of 5 with any
number typed.

- A daily limit (new cards, reviews; in a studyset's study options and the
  study settings) is a switch, "Limit new cards per day", and its number
  while on. Counting down to 0 for no limit had to be guessed, and a goal
  has nothing like it. Switched off and on again, the limit comes back as
  it was; the first time, at the defaults' 20 new cards, or 200 reviews as
  Anki starts them. Stored as before: 0 still stands for none.
- The study goal counts in steps of 5 as the limits do (reviews: 25).
  Plus and minus go to the next multiple, so 12 steps to 15 or 10, never
  below the first step.
- Tapping the number lets any other be typed, from 1 (CountStepper.onTyped).

Pomodoro minutes and cards per round keep their own steps.

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_de.arb

@@ -883,6 +883,9 @@
   "studySortNewestFirst": "Neueste zuerst",
   "studyFewer": "Verringern",
   "studyMore": "Erhöhen",
+  "countStepperType": "Zahl eingeben",
+  "studyLimitNewPerDay": "Neue Karten pro Tag begrenzen",
+  "studyLimitReviewsPerDay": "Wiederholungen pro Tag begrenzen",
 
   "studysetMenuReset": "Fortschritt zurücksetzen",
   "studysetResetTitle": "Fortschritt zurücksetzen?",