i18n: improve Spanish translation quality and consistency

[independent-arg]

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Hello! I have completely revised and updated (without AI) the Spanish (Spain) translation for UniGetUI.

The previous translation had several issues that could affect the user experience, so I have focused on the following improvements:

• Removed duplicate translations: fixed instances where different keys had the same translation by mistake, ensuring each string is unique and contextually accurate.

• Corrected grammar and spelling: fixed various typos and missing accents throughout the file.

• Unified tone and style: the previous version switched between formal and informal address. I have unified the tone to be consistent.

• Updated technical terminology: replaced literal translations (calques) with standard software terminology in Spanish (e.g., using "Guardar" instead of "Salvar", and "Cuadrícula" instead of "Grilla").

• Parity with the en.json file.

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[independent-arg]

I am also in the process of improving the Mexican Spanish version, as it contains the same problems as this translation.