refactor(gui,config): rename file path fields and split BIN selector

- Rename bit_path -> bootloader_bit_path for clarity
- Rename elf_path -> bootloader_elf_path for clarity
- Split single bin_path into bootloader_bin_path (flash) and
  firmware_bin_path (TFTP upload)
- Reorder Configuration panel: BIT -> ELF -> Bootloader BIN ->
  Firmware BIN
- Update display names with file extensions and context labels
- Update all step references (Step 2 flash, Step 3 bootloader,
  Step 4 TFTP) to use new field names

config/default_config.yaml

@@ -13,9 +13,16 @@ serial_port: ""
 serial_baudrate: 115200
 
 # File paths (relative to config file directory)
-bin_path: ""
-elf_path: ""
-bit_path: ""
+
+# Bootloader files (Step 3: Load Bootloader)
+bootloader_bit_path: ""
+bootloader_elf_path: ""
+
+# Flash programming (Step 2: Program & Verify Flash)
+bootloader_bin_path: ""
+
+# Firmware upload (Step 4: Load Main Program via TFTP)
+firmware_bin_path: ""
 
 # Reboot settings
 reboot_timeout: 30