✨ feat: Xilinx Kit directory selector + multi-version scan

Configuration panel now has a 'Xilinx Kit:' row at the top with:
- Entry field showing current path
- [Browse] button using system directory dialog (cross-platform)
- Select root directory (e.g. /data/xilinx or C:/Xilinx)

vitis_checker.py now scans xilinx_root subdirectories:
  1. System PATH (shutil.which) — works on all platforms
  2. Legacy vitis_path/bin
  3. xilinx_root/Vitis/*/bin and Vivado/*/bin (all versions, newest first)
  4. Common install paths (Windows: C:/Xilinx, Linux: /data/xilinx, /opt/xilinx)

Multi-version: scans all version directories under Vitis/ and Vivado/,
picks the newest version for each tool.

Config: xilinx_path replaces vitis_path (backward compat kept)

config/default_config.yaml

@@ -1,8 +1,10 @@
 # Default configuration for Zynq XC7Z100 Flasher
 # Paths are relative to this config file's directory.
 
-# Vitis/Vivado installation path (where bootgen, impact, etc. reside)
-vitis_path: ""
+# Xilinx installation root (e.g., /data/xilinx or C:/Xilinx)
+# The program scans subdirectories for Vitis/Vivado versions.
+# If empty, tools are searched via system PATH.
+xilinx_path: ""
 
 # Zynq device network settings
 zynq_ip: "192.168.100.11"