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.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