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>

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "stegasoo"
-version = "4.1.1"
+version = "4.1.2"
 description = "Secure steganography with hybrid photo + passphrase + PIN authentication"
 readme = "README.md"
 license = "MIT"