fix(i18n): apply missing translations to MobileNav component

[pranjal270]

Issue

Fixes #3928

Some MobileNav UI labels ("My Stuff", "Settings", "Log Out", "by…") were hardcoded and did not respond to language changes. Additionally, the corresponding translation keys — including "Select Language" — were missing from non-English locale files.

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Demo

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Changes

• Replaced hardcoded strings in MobileNav.jsx with t() calls.
• Added missing translation keys to all locale JSON files using English values as placeholders.
• Added a script to sync missing keys from the base English translations file.
• Added verified Hindi translations for key labels for local testing.

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I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• has no typecheck errors (npm run typecheck)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number.
• meets the accessibility guidelines.

[yugalkaushik]

@pranjal270 Thank you for the PR. Please review the contribution guidelines, as we prefer translation changes only in languages you are proficient in or native to. Additionally, very large translation updates are generally not preferred since they are difficult to review properly.