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

src/main.lua

@@ -3,7 +3,10 @@
 -- Dragons@Work Digital Sovereignty Project
 
 local socket = require("socket")
-local cjson = require("cjson")
+local found_cjson, cjson = pcall(require, 'cjson')
+if not found_cjson then
+    cjson = require('dkjson')
+end
 
 -- Load modules
 local Auth = require("src.auth")