Require capitalization and final punctuation again; add a UI reminder

Follow-up to the previous commit: testing further showed a missing
initial capital also fails validation, which raised the real
question (CLAUDE.md: ask rather than decide alone on an ambiguous
pedagogical point) — is sentence-initial capitalization and final
punctuation something the audio should convey, or a general writing
convention the child applies regardless of what they hear?

Decided: a general convention, like in a real classroom dictée. So
both stay required, and the terminal-punctuation tolerance added in
the previous commit is reverted (DictationAnalyzer back to comparing
the text as-is).

Since the audio still can't vocalize either rule, the child needs to
be told some other way: added a permanent reminder label to the
dictation scene ("N'oublie pas : une majuscule au début de la phrase
et un point à la fin !"), sourced from data/localization/fr.json.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

data/localization/fr.json

@@ -38,6 +38,7 @@
   "feedback_panel.correction_label": "La bonne réponse :",
   "sentence_correction.correction_placeholder": "Corrige ce mot",
   "dictation.instruction_label": "Écris la phrase que tu entends :",
+  "dictation.convention_hint": "N'oublie pas : une majuscule au début de la phrase et un point à la fin !",
   "dictation.input_placeholder": "Écris ici...",
   "dictation.replay_button": "Réécouter",
   "dictation.provisional_audio_badge": "Voix provisoire",