Clarify .env vs sky-cam.conf split for toggles and private values

Rule is now consistent throughout:
- *_ENABLED flags are plain true/false in sky-cam.conf — flip them there
- Private values (URLs, credentials, location) go in .env only

sky-cam.conf:
- NTFY_ENABLED, EMAIL_ENABLED, MM_NOTIFY_ENABLED are now plain false (not
  using the ${VAR:-} pattern, which implied they could go in .env)
- NTFY_URL, EMAIL_TO, EMAIL_FROM, MM_NOTIFY_CHANNEL_ID use ${VAR:-} pattern
  so they are set in .env
- LATITUDE/LONGITUDE comment clarified with example .env lines

install.sh .env.example: updated notification comment to match

https://claude.ai/code/session_01C4jbd3waXG3eKZYbGUjLUQ

install.sh

@@ -280,10 +280,11 @@ env_example="$HERE/.env.example"
     echo "channel_id=your-daily-upload-channel-id-here"
     echo ""
     echo "# ── Notifications ───────────────────────────────────────────────────────────"
-    echo "# Also set NTFY_ENABLED=true / EMAIL_ENABLED=true / MM_NOTIFY_ENABLED=true"
-    echo "# in sky-cam.conf to activate the method."
+    echo "# Set the private values here, then flip the matching *_ENABLED toggle"
+    echo "# to true in sky-cam.conf."
     echo "#NTFY_URL=https://ntfy.sh/your-topic"
     echo "#EMAIL_TO=you@example.com"
+    echo "#EMAIL_FROM=skycam@localhost"
     echo "#MM_NOTIFY_CHANNEL_ID=your-notify-channel-id-here"
 } > "$env_example"
 echo "  wrote .env.example  (cp .env.example .env  then fill in real values)"