fix: Standardize Trial Balance table styling and add export functionality

- Refactor table columns from grouped (Debit+Credit vertical) to separate columns for better scannability
- Replace custom HTML buttons with PrimeVue Button components (icon + label)
- Move filter action buttons to separate row below filters (matches InvoicesView pattern)
- Add Excel and PDF export functionality that fetches ALL data (not just current page)
- Update CSS_PATTERNS.md with unified table column structure and filter button patterns
- Update CLAUDE.md with table styling requirements and anti-patterns

This ensures visual consistency across all table views in the application.

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Co-Authored-By: Claude <noreply@anthropic.com>

CLAUDE.md

@@ -189,6 +189,18 @@ ttl_your_data: int = int(os.getenv('CACHE_TTL_YOUR_DATA', '600'))  # 10 min defa
 - ❌ Never use `:deep()` in components (use `src/assets/css/vendor/` for PrimeVue overrides)
 - ❌ Never duplicate CSS (write once, use everywhere)
 
+**Tables - Unified Column Structure & Filter Buttons**:
+- ✅ **ALWAYS use separate columns** for related data (Debit | Credit, not Debit+Credit stacked)
+- ✅ **Use PrimeVue DataTable** with one value per `<Column>` component
+- ✅ **Add filter/action buttons** (clear, export Excel, export PDF, refresh) **on separate row below filters**
+- ✅ **PrimeVue Button** components with **icon + label** (not icon-only!)
+- ✅ **Export ALL data** from backend (page_size: 999999), not just current page
+- ❌ **Never group multiple values** vertically in a single column
+- ❌ **Never use HTML `<button>` for filter actions** (use PrimeVue `<Button>`)
+- ❌ **Never put action buttons on same row as filters** (separate row!)
+- ❌ **Never export only current page** (must fetch all data for export)
+- 📖 **See**: `CSS_PATTERNS.md` → Table Patterns → Unified Table Column Structure & Filter/Action Buttons
+
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