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/en.js

@@ -637,4 +637,6 @@ export default {
   "keys.budgetOf": "budget {b}/mo",
   "dash.kpi.orBalance": "OpenRouter balance",
   "keys.h.install": "Install:",
+  "keys.snip.keyInputPh": "paste your API key",
+  "keys.snip.apply": "Insert key",
 };