Client does not implement refresh token handling, causing unnecessary logouts

## Description

The server correctly implements refresh token support (issues refresh tokens on login, has `/auth/refresh` endpoint), but the client never uses refresh tokens. This causes users to be logged out after the access token expires (default 2 hours) even though their refresh token is still valid (default 7 days).

## Current Behavior

1. User logs in - server returns `token` and `refreshToken`
2. Client stores only `token`, ignores `refreshToken`
3. After 2 hours, access token expires
4. Client receives 401, immediately clears auth and redirects to login
5. User must log in again

## Expected Behavior

1. User logs in - server returns `token` and `refreshToken`
2. Client stores both tokens
3. After 2 hours, access token expires
4. Server returns 403 "Request new access token"
5. Client calls `POST /auth/refresh` with refresh token
6. Client receives new access token, retries original request
7. User session continues seamlessly for 7 days

## Affected Files

- `client/src/Features/Auth/authSlice.js` - Only stores `token`, not `refreshToken`
- `client/src/Utils/NetworkService.js` - Never sends `x-refresh-token` header, no refresh logic on 401/403

## Proposed Fix

I have a working fix and will open a PR.

Changes:
- Store `refreshToken` in Redux state on login/register
- Send `x-refresh-token` header with all requests
- On 403 response, call `/auth/refresh` to get new token
- Retry failed requests with new token
- Queue concurrent requests during refresh to prevent race conditions

## Environment

- Server version: latest (docker image `bluewaveuptime/uptime_server:latest`)
- Client version: latest (docker image `bluewaveuptime/uptime_client:latest`)

Start with client/src/Features/Auth/authSlice.js to trace login and registration token state, then inspect client/src/Utils/NetworkService.js for request, 401, and 403 handling. Verify the existing /auth/refresh contract and the x-refresh-token header behavior. Done means refresh tokens are retained and failed requests can recover without unnecessary logout, including concurrent requests during refresh.

Written by the indexing model from the issue text.