Update TransactionsPage to match castle extension date range API changes

Synchronized TransactionsPage with castle LNbits extension API updates that introduced custom date range filtering.

**API Changes (ExpensesAPI.ts):**
- Updated `getUserTransactions()` to support `start_date` and `end_date` parameters (YYYY-MM-DD format)
- Custom date range takes precedence over preset days parameter
- Updated comment: default changed from 5 to 15 days, options now 15/30/60 (removed 5 and 90)

**UI Changes (TransactionsPage.vue):**
- **New defaults**: Changed default from 5 days to 15 days
- **New preset options**: 15, 30, 60 days (removed 5 and 90 day options)
- **Custom date range**: Added "Custom" option with date picker inputs
  - From/To date inputs using native HTML5 date picker
  - Apply button to load transactions for custom range
  - Auto-clears custom dates when switching back to preset days
- **State management**:
  - `dateRangeType` ref supports both numbers (15, 30, 60) and 'custom' string
  - `customStartDate` and `customEndDate` refs for custom date range
- **Smart loading**: Only loads transactions after user provides both dates and clicks Apply

**Priority Logic:**
- Custom date range (start_date + end_date) takes precedence
- Falls back to preset days if custom not selected
- Defaults to 15 days if custom selected but dates not provided

Matches castle extension implementation exactly (see castle extension git diff in fava_client.py, views_api.py, and static/js/index.js).

🐢 Generated with [Claude Code](https://claude.com/claude-code)

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

src/modules/expenses/services/ExpensesAPI.ts

@@ -404,7 +404,9 @@ export class ExpensesAPI extends BaseService {
     options?: {
       limit?: number
       offset?: number
-      days?: number // 5, 30, 60, or 90
+      days?: number // 15, 30, or 60 (default: 15)
+      start_date?: string // ISO format: YYYY-MM-DD
+      end_date?: string // ISO format: YYYY-MM-DD
       filter_user_id?: string
       filter_account_type?: string
     }
@@ -415,7 +417,15 @@ export class ExpensesAPI extends BaseService {
       // Add query parameters
       if (options?.limit) url.searchParams.set('limit', String(options.limit))
       if (options?.offset) url.searchParams.set('offset', String(options.offset))
-      if (options?.days) url.searchParams.set('days', String(options.days))
+
+      // Custom date range takes precedence over days
+      if (options?.start_date && options?.end_date) {
+        url.searchParams.set('start_date', options.start_date)
+        url.searchParams.set('end_date', options.end_date)
+      } else if (options?.days) {
+        url.searchParams.set('days', String(options.days))
+      }
+
       if (options?.filter_user_id)
         url.searchParams.set('filter_user_id', options.filter_user_id)
       if (options?.filter_account_type)