fix(screenshots): harden event-triggered MQTT screenshot flow and cleanup docs

- fix race where periodic captures could overwrite pending event_start and event_stop metadata before simclient published
- keep latest.jpg and meta.json synchronized so triggered screenshots are not lost
- add stale pending trigger self-healing to recover from old or invalid metadata states
- improve non-interactive capture reliability with DISPLAY and XAUTHORITY fallbacks
- allow periodic idle captures in development mode so dashboard previews stay fresh without active events
- add deeper simclient screenshot diagnostics for trigger and metadata handling
- add regression test script for metadata preservation and trigger delivery
- add root-cause and fix documentation for the screenshot MQTT issue
- align and deduplicate README screenshot and troubleshooting sections; update release notes to March 2026
- fix scripts/start-dev.sh .env loading to ignore comments safely and remove export invalid identifier warnings

scripts/start-dev.sh

@@ -1,5 +1,14 @@
 #!/bin/bash
+set -e
+
 cd "$(dirname "$0")/.."
 source venv/bin/activate
-export $(cat .env | xargs)
+
+# Load .env in a shell-safe way (supports comments and quoted values).
+if [ -f .env ]; then
+	set -a
+	source .env
+	set +a
+fi
+
 python3 src/simclient.py