Add 4.1.2 release plan

Three focused features:
1. Real progress bar for encode/decode (polling + progress file)
2. Granular decode error messages (custom exceptions, specific UI feedback)
3. Mobile-responsive polish (touch targets, stacked layouts, camera hints)

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

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

PLAN-4.1.2.md

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+# Stegasoo 4.1.2 Plan
+
+## Release Theme
+Polish and UX improvements after the 4.1.1 stability release.
+
+---
+
+## 1. Real Progress Bar for Encode/Decode
+
+**Status:** Planned
+
+**Problem:** Users see elapsed time but no indication of how far along the operation is. Long DCT encodes on Pi can take 2-3 minutes with no feedback.
+
+**Solution:** Polling + progress file approach
+
+### Backend Changes
+
+1. **dct_steganography.py** - Write progress during block loop:
+   ```python
+   if progress_file and block_num % 50 == 0:
+       with open(progress_file, 'w') as f:
+           json.dump({"current": block_num, "total": total_blocks, "phase": "embedding"}, f)
+   ```
+
+2. **app.py** - New endpoints:
+   - `POST /encode` returns `job_id`, starts subprocess
+   - `GET /encode/progress/<job_id>` returns progress JSON
+   - `GET /encode/result/<job_id>` returns final result when done
+
+3. **Subprocess wrapper** - Pass progress file path to encode/decode functions
+
+### Frontend Changes
+
+1. **stegasoo.js** - After form submit:
+   - Show progress bar (Bootstrap progress component)
+   - Poll `/encode/progress/{job_id}` every 500ms
+   - Update bar width and percentage text
+   - Show phase (hashing, embedding, encoding, etc.)
+
+2. **Templates** - Add progress bar markup to encode.html and decode.html
+
+### Files to Modify
+- `src/stegasoo/dct_steganography.py`
+- `frontends/web/app.py`
+- `frontends/web/static/js/stegasoo.js`
+- `frontends/web/templates/encode.html`
+- `frontends/web/templates/decode.html`
+
+---
+
+## 2. Granular Decode Error Messages
+
+**Status:** Planned
+
+**Problem:** Decode failures show generic "Decryption failed" - users don't know if it's wrong photo, wrong passphrase, wrong PIN, corrupted image, or format mismatch.
+
+**Solution:** Bubble up specific error types from library to UI
+
+### Library Level (`src/stegasoo/`)
+
+1. **Custom exception classes:**
+   ```python
+   class StegasooError(Exception): pass
+   class InvalidMagicBytesError(StegasooError): pass
+   class DecryptionError(StegasooError): pass
+   class ReedSolomonError(StegasooError): pass
+   class PayloadTooLargeError(StegasooError): pass
+   class InvalidHeaderError(StegasooError): pass
+   class NoDataFoundError(StegasooError): pass
+   ```
+
+2. **Raise specific exceptions** in decode paths:
+   - Magic bytes mismatch → "Not a Stegasoo image or wrong mode (LSB/DCT)"
+   - RS decode failure → "Image corrupted beyond repair"
+   - AES-GCM auth fail → "Wrong credentials (photo/passphrase/PIN)"
+   - Header parse fail → "Invalid or corrupted header"
+   - No stego data → "No hidden data found in image"
+
+3. **Error codes** for programmatic handling:
+   ```python
+   class ErrorCode(Enum):
+       INVALID_MAGIC = "invalid_magic"
+       DECRYPTION_FAILED = "decryption_failed"
+       RS_FAILED = "rs_failed"
+       # etc.
+   ```
+
+### Web UI Level (`frontends/web/`)
+
+1. **app.py** - Catch specific exceptions, return error type:
+   ```python
+   except InvalidMagicBytesError:
+       flash("This doesn't appear to be a Stegasoo image, or mode mismatch", "danger")
+   except DecryptionError:
+       flash("Wrong credentials - check reference photo, passphrase, and PIN", "warning")
+   ```
+
+2. **decode.html** - Error-specific help text:
+   - Wrong credentials → "Double-check your reference photo matches exactly"
+   - Corrupted → "Image may have been re-saved or compressed"
+   - Mode mismatch → "Try switching between Auto/DCT/LSB"
+
+### Files to Modify
+- `src/stegasoo/__init__.py` (export exceptions)
+- `src/stegasoo/exceptions.py` (new file)
+- `src/stegasoo/dct_steganography.py`
+- `src/stegasoo/steganography.py` (LSB)
+- `frontends/web/app.py`
+- `frontends/web/templates/decode.html`
+
+---
+
+## 3. Mobile-Responsive Polish
+
+**Status:** Planned
+
+**Problem:** UI works on mobile but has rough edges - cramped buttons, hard-to-tap targets, awkward layouts on small screens.
+
+**Solution:** Targeted CSS/layout fixes for mobile breakpoints
+
+### Areas to Improve
+
+1. **Encode/Decode Forms:**
+   - Stack image drop zones vertically on mobile (currently side-by-side)
+   - Larger touch targets for file inputs
+   - Full-width buttons on small screens
+   - Passphrase input readable at smaller sizes
+
+2. **Navigation:**
+   - Hamburger menu for mobile navbar (if not already)
+   - Sticky header doesn't eat too much screen
+   - Easy thumb reach for main actions
+
+3. **Results/Output:**
+   - Download buttons full-width on mobile
+   - QR codes sized appropriately
+   - Click-to-copy message box works well with touch
+
+4. **Drop Zones:**
+   - Larger tap targets
+   - Visual feedback for touch (not just hover)
+   - Camera integration hint on mobile ("Tap to take photo or choose file")
+
+### Testing Targets
+- iPhone SE (small)
+- iPhone 14 (medium)
+- iPad (tablet)
+- Android Chrome
+
+### Files to Modify
+- `frontends/web/static/css/style.css` (or new mobile.css)
+- `frontends/web/templates/encode.html`
+- `frontends/web/templates/decode.html`
+- `frontends/web/templates/base.html` (navbar)
+
+---
+
+## Testing Checklist
+
+- [ ] Progress bar works on localhost
+- [ ] Progress bar works on Pi (slower, more visible)
+- [ ] Cancellation handling (what if user navigates away?)
+- [ ] Error states display correctly
+- [ ] Smoke test passes
+
+---
+
+## Notes
+
+- Keep 4.1.2 focused - 3 features max
+- Don't break DCT compatibility (4.1.1 RS format is stable)
+- Test on Pi before release