feat(learn): choosing how much an assistant thinks, and the assistants by provider in the field

Halkyon Learn's ADR 0021, D60 and D63:

- A chip after the scope's offers the efforts the operator allows for the
  assistant asked, its own marked; the one picked is kept per assistant on
  the device (DeviceSettings.askEfforts) and asked at, also by a
  conversation continued from the history. One the operator no longer
  allows counts as none.
- Every question asks for its effort's ceiling on output, as the thinking
  counts towards it (8,192 to 65,536 tokens; 16,384 at the model's own),
  where it asked for 4,096; the flashcards proposed, Explain and Why? ask
  at the assistant's own effort, a person's own key at none and 4,096.
- The field's picker lists the assistants under their providers' names,
  each in its colour.

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

apps/learn/halkyon_learn_flutter/lib/l10n/app_de.arb

@@ -1829,6 +1829,8 @@
   "assistantHintAsk": "{name} fragen…",
   "assistantSendAsk": "Fragen",
   "assistantScope": "Was er nachschlagen darf",
+  "assistantEffortTooltip": "Denkaufwand: wie viel er nachdenkt, bevor er antwortet",
+  "assistantEffortDefault": "{effort} (Standard)",
   "assistantScopeTicked": "Nur was ich auswähle",
   "assistantScopeStudyset": "Dieses Lernset",
   "assistantScopeTree": "Ganzer Baum",