[FEAT] Add CLDR plural form support to localization system

[PawiX25]

Summary

Add CLDR plural form support to the localization system, enabling correct plural forms for all 14 supported languages.

Closes #706

Problem

The current localization system uses a single translation string for numeric values, which results in grammatically incorrect output in languages with complex plural rules. For example, in Polish: "1 transakcji" (incorrect) instead of "1 transakcja" (correct).

Solution

Core changes (lib/l10n/flow_localizations.dart)

• Track the current locale via _currentLocale static field
• Add _pluralCategory(num n, String langCode) method implementing CLDR plural rules for all supported languages:
  • pl: one / few / many
  • ru, uk, be: one / few / many (East Slavic rules)
  • cs: one / few / other
  • ar: zero / one / two / few / many / other
  • fr, fa: one (0-1) / other
  • en, de, it, tr, es, mn: one / other
• When getTransalation() receives a num value, it resolves the CLDR plural category and looks for a suffixed key (e.g., key.one, key.few, key.many) before falling back to the base key

Translation keys

Added plural form keys for transaction counts in 11 language files:

• tabs.home.transactionsCount.{category}
• transactions.count.{category}

Languages with plural keys: en, pl, ru, uk, be, cs, ar, it, fr, es, de.
Languages skipped (nouns don't inflect after numbers): tr, mn, fa.

How it works

Translation authors add suffixed keys alongside the base key:

{
  "transactions.count": "{} transakcji",
  "transactions.count.one": "{} transakcja",
  "transactions.count.few": "{} transakcje",
  "transactions.count.many": "{} transakcji"
}

The system automatically selects the correct form based on the numeric value and current locale. If no plural key exists, it falls back to the base key — fully backward compatible.

Testing

• Verified on device with Polish locale: "1 transakcja", "2 transakcje", "5 transakcji" all display correctly
• Backward compatible: languages/keys without plural suffixes continue to work unchanged

[sadespresso]

lgtm! thank you! the backwards compat is huge.