Improve Marathi translations for clarity and natural usage

### Summary
The Marathi localization file (`lib/l10n/app_mr.arb`) contains several strings
that are literal translations or influenced by Hindi phrasing, which can feel
unnatural to native Marathi speakers.

### Expected Behavior
All Marathi strings in the localization file should sound natural,
grammatically correct, and be consistent with the English source text.

### What Needs to be Done
- Review all existing Marathi translations in `app_mr.arb`
- Correct unnatural phrasing and grammar
- Replace Hindi-influenced words with native Marathi equivalents
- Maintain the existing keys (no feature changes)
- Ensure localization builds successfully (`flutter gen-l10n` works)

### Context
I am a native Marathi speaker and noticed the issues while reviewing the
translation. This issue will track the work that improves translation quality
without altering app logic or adding new features.

### Related
Related PR: #691