refactor: Use separate .env.modern and .env.legacy files

- Remove base-legacy.yml and base-modern.yml (YAML anchors don't work with include)
- Make all compose files self-contained
- Add .env.modern (8G RAM, 4 CPU) for Win 10/11, Server 2016+
- Add .env.legacy (2G RAM, 2 CPU) for older systems
- Update README and .env.example documentation

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.env.legacy

@@ -0,0 +1,26 @@
+# Legacy Systems Configuration (Windows 7/8, Vista, XP, 2000, Server 2003-2012, Tiny)
+
+# Resources
+RAM_SIZE=2G
+CPU_CORES=2
+DISK_SIZE=32G
+
+# User Credentials
+USERNAME=Docker
+PASSWORD=admin
+
+# Language & Region
+LANGUAGE=en
+REGION=en-US
+KEYBOARD=en-US
+
+# Display
+WIDTH=1280
+HEIGHT=720
+
+# Network
+DHCP=N
+SAMBA=Y
+
+# Debug
+DEBUG=N