✨ 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

src/gui/main_window.py

@@ -52,6 +52,7 @@ from gui.widgets import (
     StatusDisplay,
     FileSelector,
     LogDisplay,
+    SubStepFrame,
 )
 
 
@@ -187,7 +188,7 @@ class MainWindow(ctk.CTk):
         self._config.bootloader_elf_path = files["bootloader_elf_path"]
         self._config.bootloader_bin_path = files["bootloader_bin_path"]
         self._config.fsbl_elf_path = files["fsbl_elf_path"]
-        self._config.erase_before_program = self._erase_cb_var.get()
+        self._config.erase_all = self._erase_cb_var.get()
         self._config.firmware_bin_path = files["firmware_bin_path"]
 
     def _save_config(self) -> None:
@@ -338,6 +339,16 @@ class MainWindow(ctk.CTk):
                 pady=(0, 2),
             )
 
+            # Step 2: add collapsible sub-step frame
+            if i == 1:
+                self._sub_step_frame = SubStepFrame(panel)
+                self._sub_step_frame.grid(
+                    row=i * 2 + 2, column=0,
+                    sticky="nsew",
+                    padx=PADDING_SMALL + 16,
+                    pady=(0, 2),
+                )
+
         # ── Progress indicator ──
         self._progress = ProgressIndicator(panel)
         self._progress.grid(
@@ -468,10 +479,10 @@ class MainWindow(ctk.CTk):
         flash_frame.grid(row=6, column=0, sticky="nsew", padx=PADDING, pady=PADDING_SMALL)
         flash_frame.grid_columnconfigure(0, weight=1)
 
-        self._erase_cb_var = ctk.BooleanVar(value=self._config.erase_before_program)
+        self._erase_cb_var = ctk.BooleanVar(value=self._config.erase_all)
         self._erase_cb = ctk.CTkCheckBox(
             flash_frame,
-            text="Erase flash before programming",
+            text="整片Flash擦除 (Erase entire flash)",
             variable=self._erase_cb_var,
             font=FONT_SMALL,
         )
@@ -748,20 +759,25 @@ class MainWindow(ctk.CTk):
             return False
 
     def _flash_callback(self, status: str, message: str) -> None:
-        """Callback for flash programming progress — tracks phases."""
+        """Callback for flash programming — drives sub-step UI."""
         if status == "phase":
-            # Track current flash phase for UI
-            self._flash_phase = message  # 'erase', 'program', 'verify', 'done'
+            self._flash_phase = message
             phase_names = {"erase": "Erasing", "program": "Programming", "verify": "Verifying", "done": "Done"}
             label = phase_names.get(message, message)
             self._log_message(f"  ▸ {label}")
+            if hasattr(self, '_sub_step_frame'):
+                self._sub_step_frame.set_phase(message)
             return
         if status == "progress":
-            pct = message.rstrip('%')
-            # Show progress within current phase
+            try:
+                pct = int(message.rstrip('%'))
+            except ValueError:
+                pct = -1
             phase_label = {"erase": "Erase", "program": "Program", "verify": "Verify"}.get(
                 self._flash_phase, "Flash")
             self._log_message(f"  [{phase_label}] {pct}%")
+            if hasattr(self, '_sub_step_frame'):
+                self._sub_step_frame.set_phase(self._flash_phase, pct)
             return
         self._log_message(f"  [{status}] {message}")
 
@@ -1018,6 +1034,12 @@ class MainWindow(ctk.CTk):
             step_idx = i - start_index
             self._set_step_status(i, "running")
 
+            # Step 2: expand sub-step frame and reset
+            if i == 1 and hasattr(self, '_sub_step_frame'):
+                self._sub_step_frame.reset()
+                self._sub_step_frame.set_expanded(True)
+                self._flash_phase = ""
+
             # Log step timeout estimate
             timeout_key, default_timeout = step_timeout_keys[i] if i < len(step_timeout_keys) else ("", 30)
             step_timeout = self._config.step_timeouts.get(timeout_key, default_timeout) if self._config else default_timeout
@@ -1032,6 +1054,10 @@ class MainWindow(ctk.CTk):
                 self._set_step_status(i, "complete" if success else "error")
                 elapsed = time.time() - t_step_start
                 self._log_message(f"  ⏱ Step {i+1} completed in {elapsed:.0f}s")
+
+                # Step 2: mark all sub-steps done
+                if i == 1 and hasattr(self, '_sub_step_frame'):
+                    self._sub_step_frame.set_phase("done")
             except Exception as e:
                 results.append(False)
                 self._set_step_status(i, "error")