[UmmmAGoodName]

Fixes #1635

Cause

index.html attached mainScript.onerror at the end of <body>, but the main bundle is a deferred <script> injected at the end of <head>. The browser starts fetching it immediately, and Firefox dispatches error for a failed deferred script as soon as the fetch fails. With a reverse proxy answering the unprefixed /scripts/main... request with a fast 404, the error arrived before the handler existed and the prefix discovery never ran.

Fix

• Bootstrap moved into <head> ahead of the injected tag, with capturing load/error listeners on document registered before the tag is parsed. They act once on the parser-inserted main script and detach themselves. (document, not window: element load events never reach window, even in the capture phase.)
• Base globals default to / up front, so a future mismatch degrades to root behaviour instead of leaving __BASE_URL__ undefined.
• Retry scripts are appended only once the body is parsed, since the bundle mounts on #app synchronously and the error can now fire mid-parse.
• Prefix walk, SRI check and __ASF_UI_LOADED__ check unchanged.

Verification

Deterministic reproduction: a small node server serves the build under /asf/, answers everything else with a fast 404, and injects a parser-blocking <script> at the start of <body> that it delays by 1.5 s. This holds the parser before the old bootstrap while the 404 for the root bundle has already arrived.

	Firefox	Chromium
old template	white page, no retry under /asf/scripts/	loads (error dispatched after parsing)
new template	loads	loads

Also verified: deep nested route (/asf/bots/x/config) walks up to /asf/, root deployment without prefix sets the defaults, dev server boots.