fix(json): add multi-platform JSON library compatibility

- Add flexible JSON detection (cjson preferred, dkjson fallback)
- Update main.lua and mail.lua with found_cjson detection
- Update start.sh to check both JSON libraries
- Enables furt to run on Arch Linux without manual patches
- Maintains API compatibility with existing cjson usage

Fixes #108

scripts/start.sh

@@ -59,10 +59,12 @@ $LUA_COMMAND -e "require('socket')" 2>/dev/null || {
     exit 1
 }
 
-$LUA_COMMAND -e "require('cjson')" 2>/dev/null || {
-    echo -e "${RED}Error: lua-cjson not found${NC}"
+# JSON library check (cjson preferred, dkjson fallback)
+if ! ($LUA_COMMAND -e "require('cjson')" 2>/dev/null || $LUA_COMMAND -e "require('dkjson')" 2>/dev/null); then
+    echo -e "${RED}Error: No JSON library found${NC}"
+    echo "Install lua-cjson or lua-dkjson"
     exit 1
-}
+fi
 
 cd "$PROJECT_DIR"