feat(keys): harness snippets for every OS + install command when missing

The harness auto-setup scripts were bash-only (heredocs, mkdir -p) — useless
on Windows. Every scripty snippet now exists in two flavors behind a
🐧🍎/🪟 switch (auto-detected from the browser): bash heredocs for
Linux/macOS and PowerShell here-strings ($env:, New-Item, @'..'@,
curl.exe with backtick continuations) for Windows. SDK snippets and GUI
instructions stay OS-independent.

And when detection says a harness is NOT installed on the gateway machine,
the status line now offers its one-line install command for the selected OS
(claude.ai install.sh / install.ps1, npm -g for Codex and OpenCode-on-
Windows, code --install-extension for Continue/Cline) with a copy button.
1 string ×7 languages.

web/js/i18n/en.js

@@ -636,4 +636,5 @@ export default {
   "keys.form.budgetHint": "Requests are rejected with 429 insufficient_quota once the account's month-to-date spend crosses this.",
   "keys.budgetOf": "budget {b}/mo",
   "dash.kpi.orBalance": "OpenRouter balance",
+  "keys.h.install": "Install:",
 };