Implement mnemonic phrase backup popup

[ryuash]

Description

closes #22

Checklist

• [x] Targeted PR against correct branch.
• [x] Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• [x] Wrote unit tests.
• [x] Run flutter format.
• [x] Updated the documentation.
• [x] Added an entry to the CHANGELOG.md file.
• [x] Re-reviewed Files changed in the Github PR explorer.

[codecov[bot]]

Codecov Report

Merging #56 into master will decrease coverage by 0.40%. The diff coverage is 6.68%.

@@            Coverage Diff             @@
##           master      #56      +/-   ##
==========================================
- Coverage   18.10%   17.69%   -0.41%     
==========================================
  Files         247      255       +8     
  Lines        6027     6239     +212     
==========================================
+ Hits         1091     1104      +13     
- Misses       4936     5135     +199     

Impacted Files	Coverage Δ
lib/entities/account/account_save_result.dart	87.50% <0.00%> (ø)
lib/ui/blocs/home/home_bloc.dart	0.00% <0.00%> (ø)
lib/ui/blocs/home/home_event.dart	0.00% <ø> (ø)
lib/ui/blocs/home/home_state.dart	0.00% <0.00%> (ø)
lib/ui/blocs/navigator/navigator_bloc.dart	0.00% <0.00%> (ø)
lib/ui/blocs/navigator/navigator_event.dart	0.00% <0.00%> (ø)
lib/ui/localization/export.dart	0.00% <0.00%> (ø)
lib/ui/screens/home_screen.dart	0.00% <0.00%> (ø)
...ecurity/security_backup_mnemonic_confirmation.dart	0.00% <0.00%> (ø)
...ens/security/security_login_biometrics_screen.dart	0.00% <0.00%> (ø)
... and 26 more

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