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>

PLAN-4.1.2.md

@@ -7,7 +7,7 @@ Polish and UX improvements after the 4.1.1 stability release.
 
 ## 1. Real Progress Bar for Encode/Decode
 
-**Status:** Planned
+**Status:** Done
 
 **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.
 
@@ -76,7 +76,7 @@ Polish and UX improvements after the 4.1.1 stability release.
 
 ## 3. Mobile-Responsive Polish
 
-**Status:** Planned
+**Status:** Done
 
 **Problem:** UI works on mobile but has rough edges - cramped buttons, hard-to-tap targets, awkward layouts on small screens.
 
@@ -203,7 +203,7 @@ Polish and UX improvements after the 4.1.1 stability release.
 
 ## 8. Release Validation Script
 
-**Status:** Planned
+**Status:** Done
 
 **Problem:** Manual release checklist is error-prone. Need automated validation.
 
@@ -245,6 +245,6 @@ Polish and UX improvements after the 4.1.1 stability release.
 
 ## Notes
 
-- Keep 4.1.2 focused - 9 features (6 done)
+- Keep 4.1.2 focused - 9 features (9 done)
 - Don't break DCT compatibility (4.1.1 RS format is stable)
 - Test on Pi before release