docs: add whatsnew.md release notes for the upcoming main merge

User-facing summary of changes since fd2b3d0 grouped into New features
(custom analyzers, apply-path button, DB Optimize button, auto retention,
sluggish-device watchdog), Reliability & polish (polling-only fix for
load freezes, channel cache, scope-key refresh, multi-byte path
rendering, TCP self-heal, region scope on slots >7, console parser),
and a deploy note about restarting the host watchdog. Also linked from
user-guide.md's Getting Help section. File is meant to be refreshed
before each merge to main.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docs/user-guide.md

@@ -845,6 +845,7 @@ To get the full PWA experience with app badge counters:
 
 ## Getting Help
 
+- **What's New:** [whatsnew.md](whatsnew.md)
 - **Full README:** [README.md](../README.md)
 - **Contact Management Guide:** [contact-management.md](contact-management.md)
 - **Repeater Management:** [rpt-mgmt.md](rpt-mgmt.md)