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/arcane/.env.example

@@ -1,11 +1,19 @@
 # Arcane Stack Environment Variables
 # These variables are used for Arcane Docker Management UI
 
-# Base Configuration
-DOMAIN=example.duckdns.org
-SERVER_HOSTNAME=myserver
-TZ=America/New_York
+# Server Configuration
+SERVER_IP=
+
+# User Configuration
+PUID=
+PGID=
 
 # Directory Configuration
-STACKS_DIR=/opt/stacks
-PROJECTS_DIR=/opt/stacks
+PROJECTS_DIR=
+
+# Arcane Configuration
+ARCANE_ENCRYPTION_KEY=
+ARCANE_JWT_SECRET=
+
+# Optional: Agent Configuration
+ARCANE_AGENT_TOKEN=