fix(1169): focus for input unit keeps resetting while typing

[Sim-sat]

Description

What problem does this PR solve?
The input field kept resetting the focus making it hard to type.

How did you implement the solution?
Using a ref for focus.

Linked Issue: Closes #1169

How to Test

1. Add food to diary
2. Enter quanity
3. Verify input is not bugged

PR Type

• Issue (bug fix)
• New Feature
• Refactor
• Documentation

Checklist

All PRs:

• [MANDATORY - ALL] Integrity & License: I certify this is my own work, free of malicious code, and I agree to the License terms.

New features only:

• [MANDATORY for new feature] Alignment: I have raised a GitHub issue and it was reviewed/approved by maintainers or it was approved on Discord.

Frontend changes (SparkyFitnessFrontend/ or src/):

• [MANDATORY for Frontend changes] Quality: I have run pnpm run validate and it passes.
• [MANDATORY for Frontend changes] Translations: I have only updated the English (en) translation file.

Backend changes (SparkyFitnessServer/):

• [MANDATORY for Backend changes] Code Quality: I have run typecheck, lint, and tests. New files use TypeScript, new endpoints have Zod schemas, and new endpoints include tests.
• [MANDATORY for Backend changes] Database Security: I have updated rls_policies.sql for any new user-specific tables.

UI changes (components, screens, pages):

• [MANDATORY for UI changes] Screenshots: I have attached Before/After screenshots below.

[gemini-code-assist]

Code Review

This pull request refactors the focus management in the FoodUnitSelector component by replacing a callback-based ref with a useRef and a useEffect hook. This ensures the quantity input is focused and selected only when the dialog opens or the loading state changes, preventing focus issues during typing. I have no further feedback to provide.