serve webdav,http: fix directory Last-Modified using --default-time fallback - fixes #9905 - #9906

What does this change do?

Backends without real directory metadata (Swift, S3-style buckets, memory)
list directories as synthesized pseudo-entries with an unknown modtime.
fs.Dir.ModTime() falls back to the configured --default-time (default
2000-01-01) whenever the real modtime is unknown, and this frozen sentinel
was being passed straight through as the Last-Modified header in serve http directory listings, and as the PROPFIND getlastmodified /
OwnCloud lastmodified properties in serve webdav. Because the value
never changes, reverse proxies and WebDAV clients that use Last-Modified
for cache validation see the same date forever and can keep serving stale
directory listings indefinitely.

This makes both servers substitute the current time for the
--default-time sentinel, but only when the entry being served is a
directory — files, and any directory with a real known modtime, are
unaffected.

Repro, verified with and without the fix (using the :memory: backend,
which synthesizes zero-modtime directories the same way Swift/S3 do):

$ rclone serve http :memory: --addr :8080 &
$ echo hi | rclone rcat :memory:subdir/hello.txt
$ curl -I http://localhost:8080/subdir/
Last-Modified: Sat, 01 Jan 2000 00:00:00 GMT   # before the fix
Last-Modified: Fri, 11 Sep 2026 08:22:31 GMT   # after the fix

Testing done:

• go build ./..., go vet ./..., golangci-lint run ./cmd/serve/http/... ./cmd/serve/webdav/... (0 issues)
• go test ./cmd/serve/http/... and go test ./cmd/serve/webdav/..., including two new tests using the :memory: backend to reproduce a zero-modtime directory without needing Swift credentials: TestDirectoryListingLastModifiedUnknownModTime (serve http) and TestPROPFINDLastModifiedUnknownModTime (serve webdav, PROPFIND). Both verified to fail on unpatched code and pass with the fix.
• Live reproduction: built rclone from source, ran rclone serve http :memory: and rclone serve webdav :memory:, uploaded a file into a subdirectory, and confirmed the directory's Last-Modified/getlastmodified value before and after the fix as shown above.

Linked issue

Fixes #9905

Checklist

• This change is trivial OR it has been discussed and agreed in the linked issue.
• I have read the contribution guidelines.
• (If I used AI tools to help write this code) I have read and understood the AI-assisted contributions guidance, and I have tested and take ownership of this change myself.
• I have added tests for all changes in this PR if appropriate.
• I have added documentation for the changes if appropriate. (none needed — no flag/option added or changed)
• All commit messages are in house style.
• (Backend changes only) test_all passes for this backend... (not applicable — not a backend change)
• This Pull Request is ready for review.