fix: Resolve all lint warnings and type errors across frontend and backend.

Frontend:
- Memoize pagination object in TransactionsTable to prevent unnecessary re-renders and fix exhaustive-deps warning
- Add optional success and message fields to backup API response types for proper error handling

Backend:
- Add TypedDict for transaction type configuration to improve type safety in generate_sample_db
- Fix unpacking of amount_range with explicit float type hints
- Add explicit type hints for descriptions dictionary and specific_descriptions variable
- Fix sync endpoint return types: get_sync_status returns SyncStatus and sync_now returns SyncResult
- Fix transactions endpoint data type declaration to properly support Union types in PaginatedResponse

All checks now pass:
- Frontend: npm lint and npm build ✓
- Backend: mypy type checking ✓
- Backend: ruff lint on modified files ✓

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

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

leggen/commands/generate_sample_db.py

@@ -5,11 +5,19 @@ import random
 import sqlite3
 from datetime import datetime, timedelta
 from pathlib import Path
-from typing import Any
+from typing import Any, TypedDict
 
 import click
 
 
+class TransactionType(TypedDict):
+    """Type definition for transaction type configuration."""
+
+    description: str
+    amount_range: tuple[float, float]
+    frequency: float
+
+
 class SampleDataGenerator:
     """Generates realistic sample data for testing Leggen."""
 
@@ -42,7 +50,7 @@ class SampleDataGenerator:
             },
         ]
 
-        self.transaction_types = [
+        self.transaction_types: list[TransactionType] = [
             {
                 "description": "Grocery Store",
                 "amount_range": (-150, -20),
@@ -227,6 +235,8 @@ class SampleDataGenerator:
                 )[0]
 
                 # Generate transaction amount
+                min_amount: float
+                max_amount: float
                 min_amount, max_amount = transaction_type["amount_range"]
                 amount = round(random.uniform(min_amount, max_amount), 2)
 
@@ -245,7 +255,7 @@ class SampleDataGenerator:
                 internal_transaction_id = f"int-txn-{random.randint(100000, 999999)}"
 
                 # Create realistic descriptions
-                descriptions = {
+                descriptions: dict[str, list[str]] = {
                     "Grocery Store": [
                         "TESCO",
                         "SAINSBURY'S",
@@ -273,7 +283,7 @@ class SampleDataGenerator:
                     "Transfer to Savings": ["SAVINGS TRANSFER", "INVESTMENT TRANSFER"],
                 }
 
-                specific_descriptions = descriptions.get(
+                specific_descriptions: list[str] = descriptions.get(
                     transaction_type["description"], [transaction_type["description"]]
                 )
                 description = random.choice(specific_descriptions)