v4.1.5: Accordion UI, webcam QR scanning, Pi image fix

Encode/Decode UI:
- New accordion layout with 3 steps (encode) / 2 steps (decode)
- Gold step numbers with checkmarks on completion
- Dynamic right-aligned summaries as fields are filled
- Subtle gradient highlight on active accordion step

Webcam QR Scanning:
- Camera button for RSA key QR codes on encode/decode pages
- Camera button for channel key scanning
- 3-2-1 countdown capture for dense QR codes
- Proper scanner stop/restart on retry
- Backend decompression for STEGASOO-Z: compressed keys

RSA Key Print:
- Removed identifying text from QR print output
- Now prints plain QR code for discretion

Pi Image Script:
- Fixed 16GB resize to detect expand vs shrink
- Fresh images now properly EXPAND to 16GB
- Already-expanded images properly SHRINK to 16GB

UI Polish:
- Removed PIN helper text for compactness
- Fixed QR drop zone centering
- Fixed decode page element IDs for JS

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

frontends/web/stego_worker.py

@@ -136,14 +136,33 @@ def encode_operation(params: dict) -> dict:
     }
 
 
+def _write_decode_progress(progress_file: str | None, percent: int, phase: str) -> None:
+    """Write decode progress to file."""
+    if not progress_file:
+        return
+    try:
+        import json
+        with open(progress_file, "w") as f:
+            json.dump({"percent": percent, "phase": phase}, f)
+    except Exception:
+        pass  # Best effort
+
+
 def decode_operation(params: dict) -> dict:
     """Handle decode operation."""
     from stegasoo import decode
 
+    progress_file = params.get("progress_file")
+
+    # Progress: starting
+    _write_decode_progress(progress_file, 5, "reading")
+
     # Decode base64 inputs
     stego_data = base64.b64decode(params["stego_b64"])
     reference_data = base64.b64decode(params["reference_b64"])
 
+    _write_decode_progress(progress_file, 15, "reading")
+
     # Optional RSA key
     rsa_key_data = None
     if params.get("rsa_key_b64"):
@@ -152,6 +171,8 @@ def decode_operation(params: dict) -> dict:
     # Resolve channel key (v4.0.0)
     resolved_channel_key = _resolve_channel_key(params.get("channel_key", "auto"))
 
+    _write_decode_progress(progress_file, 25, "extracting")
+
     # Call decode with correct parameter names
     result = decode(
         stego_image=stego_data,
@@ -164,6 +185,8 @@ def decode_operation(params: dict) -> dict:
         channel_key=resolved_channel_key,  # v4.0.0
     )
 
+    _write_decode_progress(progress_file, 90, "finalizing")
+
     if result.is_file:
         return {
             "success": True,