Stop tracking site-specific dashboard logo images

KS-DMR*.png are the operator's own branding (referenced by the gitignored
dashboard/config.py LOGO_FILE), not project assets -- they were swept in by a
`git add -A` in the previous commit. Untrack them and gitignore dashboard/*.png
so they stay local.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -102,3 +102,7 @@ talkgroup_ids.*
 # Dashboard local config (copy from dashboard/config_sample.py)
 dashboard/config.py
 dashboard/__pycache__/
+
+# Site-specific dashboard branding (operator's own logo; not shipped)
+dashboard/*.png
+dashboard/*.jpg