Show "Exercise N/3" so repeated content doesn't look like a freeze

Manual report: after "Bravo!", clicking Continue seemed to do
nothing. An automated test proved the mission does advance (a new
exercise instance loads, completed_count increments) -- but
mission_02_pont_des_phrases has exactly one sentence_correction
exercise in the data right now, so the "next" exercise is the
identical sentence the child just solved. Nothing was broken; there
was just no visible signal that progress happened.

Added a persistent progress label at the top of exercise_host,
updated on every exercise load. Doesn't address the underlying
content-volume gap (still tracked in decisions.md point 14), but
makes the repetition legible instead of indistinguishable from a
stall.

Rebuilt both exports.

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

data/localization/fr.json

@@ -30,6 +30,7 @@
   "exercise_host.validate_button": "Valider",
   "exercise_host.continue_button": "Continuer",
   "exercise_host.finish_mission_button": "Terminer la mission",
+  "exercise_host.progress_label": "Exercice",
   "feedback_panel.correct_title": "Bravo !",
   "feedback_panel.incorrect_title": "Pas tout à fait",
   "feedback_panel.hint_label": "Indice :",