Fix multiple UI issues for secret display

[jhaals]

• Fix layout to properly position footer at bottom using flexbox
• Fix QR code display logic to check decrypted data length instead of encrypted secret
• Hide QR code button completely when secret is too long for QR
• Add break-words CSS class to prevent text overflow in secret display
• Increase QR code size from 180 to 250 for better readability
• Remove trailing newline from index.css

These changes improve the user experience when viewing decrypted secrets, particularly for long messages and QR code generation.

Includes all changes from #3159 contributed by @x1arch

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.68%. Comparing base (71bdbff) to head (d126950).
⚠️ Report is 63 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3209   +/-   ##
=======================================
  Coverage   81.68%   81.68%           
=======================================
  Files           8        8           
  Lines         808      808           
=======================================
  Hits          660      660           
  Misses        109      109           
  Partials       39       39           

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[jhaals]

fixes #3159