Fix Jauge always reading the object's most recent row

A Jauge could only target a whole object, not a specific record —
three gauges pointing at the same "value" field (e.g. Réputation/
Trésorerie/Confiance in one "jauge" object) all silently showed the
most recent row's value, with no way to tell them apart.

Add "Enregistrement (ligne) à suivre" (row_id) so a Jauge targets one
specific row, and "Champ contenant le nom" (champ_nom) to show a
label above the bar — both as dropdowns populated from the object's
actual fields/rows (previously "Champ numérique à afficher" was free
text the user had to type correctly by hand). No regression: without
row_id the widget still falls back to the latest row, exactly as
before.

Extract data_definition_options() (rows+fields for a definition) out
of routes/screens/screen_edit.py so controls_with_values.py can reuse
it server-side for the initial render; a small client-side handler
(bindJaugeDefinitionSelect) repopulates the same selects live when
the tracked object is changed without leaving the panel.

Verified live with Playwright: picking "jauge" then "Confiance" then
"value"/"name" in the panel renders an 80%-filled, green-leaning bar
labelled "Confiance" on /play — not the 20%/50% of the other rows.

routes/screens/screen_edit.py

@@ -31,21 +31,7 @@ def screen_edit(slug, screen_id):
     elements_by_id = {el["id"]: el for el in elements}
 
     definitions = db.list_definitions(slug)
-    definitions_data = {}
-    for d in definitions:
-        full = db.get_definition(slug, d["id"])
-        rows = db.list_rows(slug, full)
-        display_field = full["fields"][0]["name"] if full["fields"] else None
-        display_col = db.slugify(display_field).replace("-", "_") if display_field else None
-        row_options = [
-            {
-                "id": r["id"],
-                "label": f"{r.get(display_col)} (#{r['id']})" if display_col else f"Ligne #{r['id']}",
-            }
-            for r in rows
-        ]
-        field_options = [{"name": f["name"], "type": f["type"]} for f in full["fields"] if f["type"] != "relation"]
-        definitions_data[str(d["id"])] = {"rows": row_options, "fields": field_options}
+    definitions_data = {str(d["id"]): screens.data_definition_options(slug, d["id"]) for d in definitions}
 
     flow_nodes = screens.list_flow_nodes(slug, screen_id)
     flow_edges = screens.list_flow_edges(slug, screen_id)