Standardize .env.example files with complete variables and empty values

Changes:
- Reviewed all docker-compose files to identify required variables
- Updated all .env.example files to include ONLY variables used in their respective compose files
- Changed all placeholder values to empty strings for cleaner configuration
- Fixed variable name inconsistency: DOWNLOADS_DIR -> DOWNLOAD_DIR (to match main .env)
- Removed unnecessary variables that aren't used by the compose files

All .env.example files now:
- Contain only the variables actually used in the stack
- Have empty values instead of example placeholders
- Match the variable names used in docker-compose files

This ensures clean, minimal .env files for each stack without any extraneous variables or confusing placeholder values.

docker-compose/dashboards/.env.example

@@ -2,18 +2,15 @@
 # These variables are used for Homepage and Homarr
 
 # Base Configuration
-DOMAIN=example.duckdns.org
-SERVER_HOSTNAME=myserver
-TZ=America/New_York
+DOMAIN=
+TZ=
 
-# Server Configuration
-SERVER_IP=192.168.1.100
+# User Configuration
+PUID=
+PGID=
 
 # Directory Configuration
-STACKS_DIR=/opt/stacks
+STACKS_DIR=
 
 # Homepage Configuration
-HOMEPAGE_VAR_TITLE=My Homelab
-
-# Grafana Configuration (if used)
-GRAFANA_ADMIN_PASSWORD=changeme
+HOMEPAGE_ALLOWED_HOSTS=