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>

scenes/exercises/dictation.gd

@@ -3,6 +3,10 @@ extends Control
 ## par signal, ne touche ni a la sauvegarde ni a la progression. L'audio est
 ## provisoire (voir docs/specs/decisions.md, point 8) ; le badge l'indique a
 ## l'enfant tant qu'aucune voix definitive n'a remplace le fichier.
+##
+## Majuscule initiale et point final sont exiges a la correction (point 22)
+## sans etre entendus dans l'audio : le rappel ci-dessous est la facon dont
+## l'enfant en est informe, pas une tolerance ajoutee au validateur.
 
 signal answer_submitted(submitted_text: String)
 
@@ -11,6 +15,7 @@ var _dictation: Dictionary = {}
 @onready var _instruction_label: Label = %InstructionLabel
 @onready var _replay_button: Button = %ReplayButton
 @onready var _provisional_badge: Label = %ProvisionalBadge
+@onready var _convention_hint_label: Label = %ConventionHintLabel
 @onready var _text_edit: LineEdit = %TextEdit
 @onready var _validate_button: Button = %ValidateButton
 
@@ -20,6 +25,7 @@ func _ready() -> void:
     _replay_button.text = UIText.get_text("dictation.replay_button")
     _text_edit.placeholder_text = UIText.get_text("dictation.input_placeholder")
     _provisional_badge.text = UIText.get_text("dictation.provisional_audio_badge")
+    _convention_hint_label.text = UIText.get_text("dictation.convention_hint")
     _validate_button.text = UIText.get_text("exercise_host.validate_button")
 
     _replay_button.pressed.connect(_on_replay_pressed)

scenes/exercises/dictation.tscn

@@ -37,6 +37,11 @@ unique_name_in_owner = true
 layout_mode = 2
 theme_override_colors/font_color = Color(0.6, 0.45, 0.1, 1)
 
+[node name="ConventionHintLabel" type="Label" parent="Margin/VBox"]
+unique_name_in_owner = true
+layout_mode = 2
+autowrap_mode = 2
+
 [node name="ReplayButton" type="Button" parent="Margin/VBox"]
 unique_name_in_owner = true
 layout_mode = 2