✨ feat: collapsible flash sub-steps UI + erase_all checkbox

Rename erase_before_program → erase_all (semantics fix):
- erase_all: true  → -erase_all -erase_only (entire flash chip)
- erase_all: false → -erase_only (sectors matching BIN size)
Both options always erase; the checkbox controls scope, not yes/no.

UI additions:
- SubStepFrame widget: collapsible Erase/Program/Verify sub-steps
  with status dots (○/◉/●) and colored labels
- Automatically expands on Step 2 start, collapses on completion
- Phase and progress driven by real-time program_flash output parsing
- Checkbox label: '整片Flash擦除 (Erase entire flash)'

subprocess_utils.py phase detection:
- Parses 'Performing Erase/Program/Verify Operation' for phase transitions
- Emits 'phase' callbacks (erase/program/verify/done)
- Progress % tracked within current phase

config/default_config.yaml

@@ -22,13 +22,12 @@ bootloader_elf_path: ""
 # bootloader_bin_path is the bootloader image to program into QSPI flash
 # fsbl_elf_path is the FSBL (First Stage Boot Loader) required by program_flash
 # flash_type sets the QSPI interface mode (qspi-x4-single, qspi-x8-dual_parallel, etc.)
-# flash_model determines chip capacity: s25fl256s1=32MiB, s25fl128s=16MiB, etc.
-# erase_before_program: if true, erase flash sectors before programming
+# erase_all: if true, erase entire flash chip; if false, only erase sectors needed
 bootloader_bin_path: ""
 fsbl_elf_path: ""
 flash_type: "qspi-x4-single"
 flash_model: "s25fl256s1"
-erase_before_program: true
+erase_all: false
 
 # Firmware upload (Step 4: Load Main Program via TFTP)
 firmware_bin_path: ""