feat: add fullscreen TUI with illwill

- New screens: Main Menu, Compile, Run, REPL, AI Generator, AI Settings, Help
- AI configuration persisted in ~/.config/cljnim/config.json
- Navigate with arrow keys, Enter, Escape
- illwill added as dependency in cljnim.nimble

src/cljnim.nim

@@ -1,6 +1,6 @@
 # Clojure/Nim — AI-First Clojure Compiler
 import os, osproc, strutils, times
-import reader, emitter, types, repl, macros, deps, ai_assist
+import reader, emitter, types, repl, macros, deps, ai_assist, tui
 
 proc getLibPath(): string =
   let appDir = getAppDir()
@@ -206,6 +206,7 @@ proc main() =
     echo "  cljnim deps                              Resolve dependencies from deps.edn"
     echo "  cljnim -e '<code>'                       Evaluate expression"
     echo "  cljnim ai '<description>'                Generate Clojure code with AI"
+    echo "  cljnim tui                               Start interactive TUI"
     echo ""
     echo "REPL Commands:"
     echo "  :quit, :q       Exit REPL"
@@ -346,6 +347,9 @@ proc main() =
       stderr.writeLine(aiRes.suggestion)
       quit(1)
 
+  of "tui":
+    runTui()
+
   else:
     stderr.writeLine("Unknown command: " & cmd)
     quit(1)