Add French (fr) exercise instructions

Adds a tenth language to all 1,324 exercises: `instructions.fr` and
`instruction_steps.fr`, plus the schema, README, viewer and setup guide.

Register matches the existing Spanish/Italian/Polish translations —
informal second-person imperative (tutoiement):

  en: Lie flat on your back with your knees bent and feet flat on the ground.
  fr: Allonge-toi sur le dos, les genoux fléchis et les pieds à plat au sol.

How it was produced
-------------------
The 7,710 English step sentences deduplicate to 4,414 unique strings.
Those were translated (LLM-assisted, glossary-constrained, then audited),
and the per-exercise arrays were rebuilt deterministically by mapping each
English step back through that table. `instructions.fr` is the join of
`instruction_steps.fr`, so the two can never drift.

A fixed glossary pinned the recurring terms — répétitions, position de
départ, haltère/barre, sangle abdominale, omoplates, largeur des épaules —
so terminology is consistent across all 1,324 records rather than varying
sentence by sentence.

Verified
--------
- 1,324 records validate against the updated schema; a record missing `fr`
  is now rejected.
- fr step count == en step count in every record (the dataset's existing
  invariant across all nine prior languages).
- instructions.fr == ' '.join(instruction_steps.fr) in every record.
- Every pre-existing field is byte-identical; `fr` is purely additive.
- No empty strings, none left in English, no vouvoiement across 7,710 steps.
- index.html embed stays in sync with data/exercises.json; the language
  picker renders "Français" and the steps display correctly.

Note for review: these are machine translations, reviewed programmatically
rather than by a native speaker. Happy to adjust any wording.

setup.html

@@ -1401,7 +1401,7 @@ function buildLlmPrompt(fwKey, dbKey) {
 
 ## Dataset Overview
 - 1,324 fitness exercises
-- Fields: id (string, e.g. "0001"), name, category, body_part, equipment, instructions_en (full text), instructions_es (Spanish text), instructions_it (Italian text), instructions_tr (Turkish text), instructions_ru (Russian text), instructions_zh (Chinese text), instructions_hi (Hindi text), instructions_pl (Polish text), instructions_ko (Korean text), muscle_group, secondary_muscles (JSON array of strings), target, image (relative path like "images/0001.jpg"), gif_url (relative path like "videos/0001.gif"), created_at
+- Fields: id (string, e.g. "0001"), name, category, body_part, equipment, instructions_en (full text), instructions_es (Spanish text), instructions_it (Italian text), instructions_tr (Turkish text), instructions_ru (Russian text), instructions_zh (Chinese text), instructions_hi (Hindi text), instructions_pl (Polish text), instructions_ko (Korean text), instructions_fr (French text), muscle_group, secondary_muscles (JSON array of strings), target, image (relative path like "images/0001.jpg"), gif_url (relative path like "videos/0001.gif"), created_at
 
 ## Database Schema (${db.name})
 \`\`\`sql