Fix: bibliography month persists to subsequent entries without month (#3391)

Fixes #3386 (the part about the wrong months; the other part (layouting
of bibtex) is fixed in #3387)

## Summary
- Fixes incorrect month shown for bibliography entries that do not
specify a `month` in BibTeX.
- Ensures `month`/`year` do not persist between entries in
`_layouts/bib.liquid`.

## Root Cause
- In Liquid, captured variables persist across iterations if not
reinitialized.
- `entrymonth` and `entryyear` were only set when
`entry.month`/`entry.year` existed, causing values from a previous entry
to leak into the next one when those fields were missing.

## Changes
- Reinitialize date-related variables per entry before conditional
captures:

## Behavior
- Before: Entries without a `month` could display the previous entry's
month.
- After: Entries without a `month` display only the `year`; entries with
a `month` display the correct month and year.


<img width="1956" height="918" alt="image"
src="https://github.com/user-attachments/assets/7a9682e7-55ab-43b7-8f82-f638d14ce08e"
/>

_layouts/bib.liquid

@@ -160,9 +160,11 @@
     {% else %}
       {% capture entrytype %}{% endcapture %}
     {% endif %}
+    {% assign entrymonth = '' %}
     {% if entry.month %}
       {% capture entrymonth %}{{ " " }}{{ entry.month | capitalize }}{% endcapture %}
     {% endif %}
+    {% assign entryyear = '' %}
     {% if entry.year %}
       {% capture entryyear %}{{ " " }}{{ entry.year }}{% endcapture %}
     {% endif %}