Internal links (customMenus, project cards) don't respect baseURL when it has a subpath

[yinchi]

Expected Behavior

Internal links (e.g. a customMenus entry pointing to an internal page, or a
project card's .url pointing to an internal post) should resolve correctly
under any configured baseURL, including one with a subpath — e.g. a GitHub
Pages project site like https://user.github.io/repo/.

Current Behavior

These links are rendered as a raw, unprocessed href="{{ .url }}" with no
baseURL handling, so they only produce a correct path when baseURL has no
subpath. On a subpath deployment the link is missing the subpath entirely and
points to the wrong location — e.g. /posts instead of /repo/posts.

Two confirmed instances:

• customMenus entries (navbar.html, footer.html)
• Project card links/buttons (cards/project.html, .url) — the theme's own
  exampleSite already demonstrates an internal-path use case for this field
  (the "Rich Content" project in data/en/sections/projects.yaml), so this
  isn't hypothetical.

Possible Solution

Distinguish internal (no http:///https:// prefix) from external URLs, and
resolve internal ones by joining them onto site.BaseURL | relLangURL (the
same pattern already used ad hoc for the blog/notes nav links).

Steps to Reproduce

1. In exampleSite, note the "Rich Content" project card in
   data/en/sections/projects.yaml, which uses an internal url
   (/posts/category/sub-category/rich-content/).
2. Build with a subpath baseURL:
   hugo --gc --minify --baseURL "https://example.com/subpath/"
3. Inspect the rendered href for that project card.
4. Observe it renders as /posts/category/sub-category/rich-content — missing
   the /subpath prefix — while other internal links on the same page (e.g.
   section anchors, the post's own card elsewhere via .RelPermalink) are
   correctly prefixed with /subpath.

Screenshots

N/A — this is a server-rendered HTML output issue (wrong href value), not a
visual/layout bug.

Browsers Affected

N/A — the bug is in Hugo's build-time template rendering, not
browser-specific.

Detailed Description

Several href attributes in the theme are set directly from a data field
({{ .url }}) without passing through any Hugo URL-resolution function
(relLangURL, absLangURL, RelPermalink, etc.). This only happens to
produce a correct URL when baseURL has no subpath — on a project site or any
other non-root deployment, the link is simply wrong.