feat: bump Hugo to 0.165.0 and PaperMod to d376885

These two bumps are coupled and must land together.

PaperMod: a2eb47b (2024-09) -> d376885 (latest)
- Required for Hugo 0.146+ — the old partial path convention
  ("partials/templates/...") was made strict in 0.146 and the
  old commit breaks every page render.
- Includes 87de3fb0 (2026-05) "Implement new template system
  added with Hugo v0.146.0" plus ~15 follow-up commits
  (style/readability + extend_post_content hook).

Hugo: 0.131.0 -> 0.165.0
- Picks up 18 months of upstream work: new template system,
  output format additions, the .Site.Social deprecation that
  we already had to work around, and various bug fixes.
- `:filename` permalink token now emits a deprecation warning;
  PaperMod and our config are unaffected, will address when
  Hugo removes the alias in a future release.

Custom code (assets/css/extended/search-snippet.css,
assets/js/fastsearch.js) lives in the assets/ pipeline and is
independent of the theme, so no changes needed there.

Author: lzm

.github/workflows/hugo.yml

@@ -29,7 +29,7 @@ jobs:
       - name: Setup Hugo
         uses: peaceiris/actions-hugo@2752ce1d29631191ea3f27c23495fa06139a5b78 # v3.2.1
         with:
-          hugo-version: "0.131.0"
+          hugo-version: "0.165.0"
           extended: true
 
       - name: Build