Fix decode progress - add Argon2 phase tracking

- decode.py now writes 20% "initializing" before Argon2
- decode.py writes 25% "extracting" after Argon2 completes
- DCT extraction scales from 25-70% (was 5-70%)
- Removed duplicate "loading" writes that caused backwards jumps

Progress flow: 15% reading -> 20% initializing (Argon2) -> 25-70% extracting -> 75-95% decoding -> 100% complete

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/stegasoo/dct_steganography.py

@@ -1198,7 +1198,7 @@ def _extract_scipy_dct_safe(
     progress_file: str | None = None,
 ) -> bytes:
     """Extract using safe DCT operations with vectorized processing."""
-    _write_progress(progress_file, 0, 100, "loading")
+    # Progress starts at 25% (decode.py writes 20% for Argon2, 25% before extraction)
 
     img = Image.open(io.BytesIO(stego_image))
     width, height = img.size
@@ -1264,9 +1264,10 @@ def _extract_scipy_dct_safe(
         del blocks, dct_blocks, coeffs, quantized
         block_idx = batch_end
 
-        # Report progress (scale to 5-70% range, RS decode gets 70-100%)
+        # Report progress (scale to 25-70% range, RS decode gets 70-100%)
+        # Starts at 25% because decode.py writes 25% before calling extraction
         if progress_file and block_idx % PROGRESS_INTERVAL < BATCH_SIZE:
-            extract_pct = 5 + int(65 * block_idx / num_blocks)
+            extract_pct = 25 + int(45 * block_idx / num_blocks)
             _write_progress(progress_file, extract_pct, 100, "extracting")
 
         # Check if we have enough bits (early exit)
@@ -1382,7 +1383,7 @@ def _extract_jpegio(
     """Extract using jpegio for JPEG images."""
     import os
 
-    _write_progress(progress_file, 0, 100, "loading")
+    # Progress starts at 25% (decode.py writes 20% for Argon2, 25% before extraction)
 
     # Normalize JPEG to avoid crashes with quality=100 images
     # (shouldn't happen with stego images, but be defensive)
@@ -1397,7 +1398,7 @@ def _extract_jpegio(
         all_positions = _jpegio_get_usable_positions(coef_array)
         order = _jpegio_generate_order(len(all_positions), seed)
 
-        _write_progress(progress_file, 20, 100, "extracting")
+        _write_progress(progress_file, 30, 100, "extracting")
 
         # Try RS-protected format first (has 24-byte length prefix: 3 copies for majority voting)
         if HAS_REEDSOLO and len(all_positions) >= RS_LENGTH_PREFIX_SIZE * 8:

src/stegasoo/decode.py

@@ -8,6 +8,7 @@ Changes in v4.0.0:
 - Improved error messages for channel key mismatches
 """
 
+import json
 from pathlib import Path
 
 from .constants import EMBED_MODE_AUTO
@@ -24,6 +25,22 @@ from .validation import (
 )
 
 
+def _write_progress(progress_file: str | None, current: int, total: int, phase: str) -> None:
+    """Write progress to file for UI polling."""
+    if progress_file is None:
+        return
+    try:
+        with open(progress_file, "w") as f:
+            json.dump({
+                "current": current,
+                "total": total,
+                "percent": (current / total * 100) if total > 0 else 0,
+                "phase": phase,
+            }, f)
+    except OSError:
+        pass
+
+
 def decode(
     stego_image: bytes,
     reference_photo: bytes,
@@ -93,11 +110,17 @@ def decode(
     if rsa_key_data:
         require_valid_rsa_key(rsa_key_data, rsa_password)
 
+    # Progress: starting key derivation (Argon2 - slow on Pi)
+    _write_progress(progress_file, 20, 100, "initializing")
+
     # Derive pixel/coefficient selection key (with channel key)
     from .crypto import derive_pixel_key
 
     pixel_key = derive_pixel_key(reference_photo, passphrase, pin, rsa_key_data, channel_key)
 
+    # Progress: key derivation done, starting extraction
+    _write_progress(progress_file, 25, 100, "extracting")
+
     # Extract encrypted data
     encrypted = extract_from_image(
         stego_image,