fix(edit_meal): prevent keyboard drop; scope rebuilds; correct unit suffix; safe conversions & controller disposal

[LGKentang]

This PR fixes the issue #262

Summary

• Prevents the on-screen keyboard from dismissing while editing Base quantity.
• Reduces rebuild scope during typing for smoother UX.
• Fixes unit label logic to reflect the selected unit and metric/imperial mode.
• Uses selected unit during save-time conversions and disposes controllers properly.

What changed

• Initialize bloc once in initState; stop dispatching in build().
• BlocBuilder now uses bloc: _editMealBloc (no locator() or .add() in build).
• Removed the _baseQuantityTextController listener that called setState() on every keystroke.
• Wrapped kcal/carbs/fat/protein fields in ValueListenableBuilder(_baseQuantityTextController) so only those labels rebuild when base quantity changes.
• Replaced hardcoded " g/ml" with _unitSuffixForSelected(...).
• Save-time conversion uses selectedUnit ?? _mealEntity.mealUnit.
• Added dispose() for all TextEditingControllers.
• Set keyboardType: TextInputType.numberWithOptions(decimal: true) for Base quantity.

Why

• Dispatching InitializeEditMealEvent() inside build() caused full widget rebuilds on each keystroke → focus lost → keyboard dropped.
• Scoping rebuilds to only the fields that need updates keeps the form stable.