Improve next-word predictions

[astroboii47]

Summary

• add a small bundled English next-word phrase list
• use bundled phrase predictions as a fallback after learned next-word suggestions
• keep ordinary word completions as the final fallback so suggestion slots stay useful
• advance the prediction bar immediately after accepting a suggestion

Notes

• this PR is independent from the snippets and GIF/media picker PRs
• the bundled list is local and does not add any network dependency

Testing

• ./gradlew :app:assembleDebug

[astroboii47]

Closing in favor of #259, which contains the cleaned combined feature set with split commits.