Set Up State Management and Error Handling

## Overview
Implement comprehensive state management and error handling across the application.

## Acceptance Criteria
- [ ] authSlice: login, register, fetchMe, logout; token persist
- [ ] runnersSlice: list, create, update, remove with loading/error flags
- [ ] Axios interceptor:
- Attach bearer token
- 401 → clear state + redirect /login
- [ ] Global error normaliser (ProblemDetails → friendly message)
- [ ] Consistent loading/error UX; unauthorised paths handled gracefully

## Test Notes
- Test token persistence across page refresh
- Test 401 handling and automatic logout
- Test loading states during API calls
- Test error messages are user-friendly

## Implementation Notes
- Use Redux Toolkit for state management
- Implement proper error boundaries
- Add global loading indicators
- Ensure consistent error handling patterns

No files or test paths are named. Start by locating the existing React and Redux Toolkit setup, authentication state, and Axios client; then run the current test suite or application flow. Done means the listed auth and runner states, token persistence, 401 logout, normalized messages, loading indicators, and graceful unauthorized handling are covered.

Written by the indexing model from the issue text.