feat: overhaul category architecture with envelope budgeting and 3-level hierarchy

[MF-Rozi]

Description

What:

1. Overhauled the category system to implement a three-level hierarchical envelope budgeting model (Pillar -> Sub-Parent -> Envelope).
2. Extended Category domain entity and CategoryModel with parentId, type (expense/income), expectedMonthlyBudget, and behavioralModifier (active/passive/recurring).
3. Seeded default Root Pillars for both Expense and Income streams upon local database initialization.
4. Implemented new UI components: EnvelopeTreeListView, PortfolioDistributionCard, HierarchyInsightCard, and EnvelopeCreationForm.
5. Updated CategoryCubit and CategoryState to calculate portfolio distributions, aggregate pillar budgets, and manage category creation/editing.
6. Updated transaction entry and card widgets to adapt to the new category schema.
7. Updated and expanded unit, bloc, use case, entity, model, repository, and widget test suites.

How:

1. Replaced parentUuid with parentId (type UniqueId?) in Category and CategoryModel with appropriate Isar indexes.
2. Built reactive UI components in lib/features/category/presentation/widgets/ matching the new design specs.
3. Enhanced CategoryCubit with filtering, parent-child tree mapping, and dynamic budget calculations.
4. Migration note: Any code referencing parentUuid should migrate to parentId. Instantiating Category now requires specifying type, expectedMonthlyBudget, and behavioralModifier.

Why:

1. Supports envelope budgeting workflows with visual hierarchy and portfolio distribution insights.
2. Unifies category architecture into a structured 3-level hierarchy that supports aggregation and sub-envelope allocation.

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Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ Bug fix (non-breaking change which fixes an issue)
• ❌ Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 Code refactor
• ✅ Build configuration change
• 📝 Documentation
• 🗑️ Chore

Code Checklist

• Tested with fvm flutter test (all 97 tests passing) and verified clean analysis with fvm flutter analyze.