feat(keys): paste-your-key field — snippets carry the real key instantly

Next to the copy button: a key input and an «Insert key» button (Enter works
too). The pasted key substitutes into every harness/SDK snippet immediately
and survives tab and OS switches; a freshly created key pre-fills the field.
No more copying a snippet and then hand-editing the placeholder. 2 strings
×7 languages.

web/js/i18n/fr.js

@@ -644,4 +644,6 @@ export default {
   "keys.budgetOf": "budget {b}/mois",
   "dash.kpi.orBalance": "Solde OpenRouter",
   "keys.h.install": "Installer :",
+  "keys.snip.keyInputPh": "collez votre clé API",
+  "keys.snip.apply": "Insérer la clé",
 };