fix: add accessibility attributes, fix CSS margins, complete textAlign support, remove unused variable

- Added role="button", tabIndex, aria-label, and onKeyDown handler to verse spans for keyboard accessibility
- Fixed CSS margin/padding conflict by using py/px instead of p/margin for proper variable margin width
- Added --text-align CSS variable to getCSSVariables() and applied it in reading view
- Removed unused isTablet variable

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

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

lib/reading-preferences.ts

@@ -82,6 +82,7 @@ export function getCSSVariables(prefs: ReadingPreference): Record<string, string
     '--font-size': `${prefs.fontSize}px`,
     '--line-height': `${prefs.lineHeight}`,
     '--letter-spacing': `${prefs.letterSpacing}em`,
+    '--text-align': prefs.textAlign,
     '--bg-color': prefs.backgroundColor,
     '--text-color': prefs.textColor,
     '--margin-width': getMarginWidth(prefs.margin),