Mobile polish, release validation script, bump to v4.1.2

Mobile-responsive CSS improvements:
- Larger touch targets for drop zones and buttons (56px min)
- Touch feedback with active states for touch devices
- Camera hint text on mobile ("Tap to take photo or choose file")
- Mode buttons stack vertically on small screens
- Full-width download buttons on mobile
- Navbar doesn't stick on mobile to save screen space

Release validation script (scripts/validate-release.sh):
- Automated pre-release checks: ruff, imports, encode/decode sanity
- Optional Docker build/test (--docker flag)
- Optional Pi smoke test via SSH (--pi flag)
- Pass/fail summary with exit codes

Other:
- Version bump to 4.1.2 (pyproject.toml, constants.py, __init__.py)
- Fixed ruff import sorting in cli.py
- Updated PLAN-4.1.2.md (all 9 features complete)

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

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

src/stegasoo/__init__.py

@@ -7,7 +7,7 @@ Changes in v4.0.0:
 - encode() and decode() now accept channel_key parameter
 """
 
-__version__ = "4.0.1"
+__version__ = "4.1.2"
 
 # Core functionality
 # Channel key management (v4.0.0)

src/stegasoo/cli.py

@@ -105,6 +105,7 @@ def encode(
         stegasoo encode photo.png -r ref.jpg -f secret.pdf -o encoded.png
     """
     from PIL import Image
+
     from .encode import encode as stegasoo_encode
     from .encode import encode_file as stegasoo_encode_file
 
@@ -1108,7 +1109,9 @@ def admin_recover(db_path, password):
         stegasoo admin recover --db /path/to/stegasoo.db
     """
     import sqlite3
+
     from argon2 import PasswordHasher
+
     from .recovery import verify_recovery_key
 
     # Try default paths if not specified

src/stegasoo/constants.py

@@ -25,7 +25,7 @@ from pathlib import Path
 # VERSION
 # ============================================================================
 
-__version__ = "4.1.0"
+__version__ = "4.1.2"
 
 # ============================================================================
 # FILE FORMAT