✨ 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/subprocess_utils.py

@@ -1,7 +1,8 @@
 """Non-blocking subprocess execution with real-time output parsing.
 
 Provides run_streaming() which streams stdout/stderr to callbacks
-in real time, parsing for warnings, errors, and progress indicators.
+in real time, parsing for warnings, errors, progress indicators,
+and flash operation phase transitions (erase/program/verify).
 Used by flash_programmer and bitstream_programmer.
 """
 
@@ -15,7 +16,7 @@ from typing import Callable
 
 StreamCallback = Callable[[str, str], None] | None
 """Callback type: (category, message) where category is one of:
-   'out', 'err', 'warn', 'error', 'progress', 'done'"""
+   'out', 'err', 'warn', 'error', 'progress', 'phase', 'done'"""
 
 # Regex patterns for parsing tool output
 _WARNING_RE = re.compile(r"WARNING\s*[:\[].*", re.IGNORECASE)
@@ -24,6 +25,12 @@ _PROGRESS_RE = re.compile(r"(\d{1,3})\s*%")
 _INFO_KEYWORDS = ["connected", "downloading", "running", "initialization",
                   "verif", "write", "erase", "flash operation"]
 
+# Phase transition patterns for program_flash output
+_PHASE_ERASE_RE = re.compile(r"Performing Erase Operation", re.IGNORECASE)
+_PHASE_PROGRAM_RE = re.compile(r"Performing Program Operation", re.IGNORECASE)
+_PHASE_VERIFY_RE = re.compile(r"Performing Verify Operation", re.IGNORECASE)
+_PHASE_DONE_RE = re.compile(r"(Erase|Program|Verify) Operation successful", re.IGNORECASE)
+
 
 def run_streaming(
     cmd: list[str],
@@ -133,6 +140,24 @@ def _parse_and_callback(
         callback("warn", line)
         return
 
+    # Check for phase transitions (program_flash output)
+    if _PHASE_ERASE_RE.search(line):
+        callback("phase", "erase")
+        callback("out", line)
+        return
+    if _PHASE_PROGRAM_RE.search(line):
+        callback("phase", "program")
+        callback("out", line)
+        return
+    if _PHASE_VERIFY_RE.search(line):
+        callback("phase", "verify")
+        callback("out", line)
+        return
+    if _PHASE_DONE_RE.search(line):
+        callback("phase", "done")
+        callback("out", line)
+        return
+
     # Check for progress percentage
     m = _PROGRESS_RE.search(line)
     if m: