fix: basePath handling for assets

[fry69]

trying to fix #3391 again, this gets a bit hairy, it forces propagating the basePath through the runtime.

Well, this code may not win a beauty contest, but so far it seems work and handle the mentioned cases.

Please review, check, flame me and revert my previous basePath PRs if this path is too much of a rabbit hole 😄

[bartlomieju]

A few things I noticed:

1. Public API change to asset() and assetSrcSet() — these are exported from shared.ts and adding a basePath parameter changes the public signature. Users calling asset("/foo.css") won't break (it's optional), but it's a leaky abstraction — why should callers need to know about basePath? Internally Fresh always has access to basePath through render state, and BUILD_ID is already set as a module-level variable. Could basePath be handled the same way instead of threading it through parameters?

2. applyBasePath can double-prefix — there's no guard against path already starting with basePath. If someone does asset("/ui/foo.css") with basePath="/ui", they'd get /ui/ui/foo.css.

3. Character validation regex is incomplete — /@#?&=\s/ blocks some characters but allows others that are problematic in URL paths (e.g., %, {, }, <, >). Consider validating against a stricter URL path character set, or just doing new URL(basePath, "https://localhost") and checking it roundtrips cleanly.

4. Island CSS paths — in RemainingHead(), island CSS paths (island.css[i]) get applyBasePath applied. Are these paths guaranteed to be root-relative without the base? If they already include the base from the build step, this would double-prefix them.

[bartlomieju]

Thanks!

[bartlomieju]

Actually I'm gonna close this one since the original issue is now fixed.