feat: serve the one-line installer from the site

The site serves install.sh at /retrobios/i and install.ps1 at
/retrobios/i.ps1. generate_site.py writes both copies with the
installer address set to install.py at the commit the site is built
from (GITHUB_SHA in CI, HEAD locally), so the pinned SHA-256 and the
downloaded installer always come from one commit, including while a
deploy is still running.

Every command a user copies names the short address. The raw form
stays on the installer page as the fallback. Deploy Site now runs on
changes to install.sh and install.ps1. The install box weight keeps
the address length its ratio was fitted on, so no console page moves.

install.py

@@ -243,7 +243,9 @@ CARD_PLATFORMS = frozenset(name for name, _file, _prefix in _CARD_RULES)
 SITE_URL = "https://abdess.github.io/retrobios/"
 INSTALLER_PAGE = f"{SITE_URL}wiki/installer/"
 TROUBLESHOOTING_PAGE = f"{SITE_URL}wiki/troubleshooting/"
-BOOTSTRAP_URL = "https://raw.githubusercontent.com/Abdess/retrobios/main/install"
+# The one-line bootstraps the site serves: BOOTSTRAP_URL for sh, .ps1 after it
+# for PowerShell (scripts/sitebootstrap.py).
+BOOTSTRAP_URL = f"{SITE_URL}i"
 
 # Fallback BIOS destination per platform when --platform is forced
 # but auto-detection finds nothing on the machine.
@@ -5747,7 +5749,7 @@ class Invocation:
         if bootstrap == "ps1":
             return f"&([scriptblock]::Create((irm {BOOTSTRAP_URL}.ps1)))"
         if bootstrap == "sh":
-            return f"curl -fsSL {BOOTSTRAP_URL}.sh | {'sudo ' if elevated else ''}sh -s --"
+            return f"curl -fsSL {BOOTSTRAP_URL} | {'sudo ' if elevated else ''}sh -s --"
         python = "python" if self.os_type == "windows" else "python3"
         return f"{'sudo ' if elevated else ''}{python} install.py"