✨ feat: flash phase tracking, erase checkbox, flash_model config

Step 2 UI now shows real-time flash operation phases parsed from program_flash output:
- Erase: "Performing Erase Operation..." → sf erase → Erased: OK
- Program: "Performing Program Operation..." → write blocks 0%→100%
- Verify: "Performing Verify Operation..." → read+cmp 0%→100%

Configuration additions:
- erase_before_program checkbox: controls whether to pre-erase before programming
- flash_model: chip type for capacity reference (s25fl256s1 = 32 MiB)

subprocess_utils.py now emits 'phase' callbacks (erase/program/verify/done)
detected from program_flash output transitions.

src/flash_programmer.py

@@ -279,7 +279,11 @@ def full_flash_program(
     bin_path: str | Path,
     callback: ProgressCallback = None,
 ) -> list[FlashResult]:
-    """Execute full flash workflow: wipe → program → verify.
+    """Execute full flash workflow: (optional wipe) → program → verify.
+
+    If config.erase_before_program is True, erases flash sectors before
+    programming. Otherwise, program_flash handles sector erase during
+    programming (per-sector, not full chip).
 
     Args:
         config: Application configuration.
@@ -291,10 +295,17 @@ def full_flash_program(
     """
     results: list[FlashResult] = []
 
-    # Step 1: Erase the entire flash
-    results.append(wipe_flash(config, callback))
-    if not results[-1].success:
-        return results
+    # Step 1: Erase (optional — controlled by config.erase_before_program)
+    if getattr(config, 'erase_before_program', True):
+        results.append(wipe_flash(config, callback))
+        if not results[-1].success:
+            return results
+    else:
+        results.append(FlashResult(
+            step="wipe",
+            success=True,
+            message="Erase skipped (handled by program_flash)",
+        ))
 
     # Step 2: Program and verify (program_flash -verify)
     results.append(program_flash_bin(config, bin_path, callback))