refactor: remove all futures-related capabilities

Delete USDT-M futures support since the user's Binance API key does not
support futures trading. This simplifies the CLI to spot-only:

- Remove futures client wrapper (um_futures_client.py)
- Remove futures trade commands and close position logic
- Simplify account service to spot-only (no market_type field)
- Remove futures references from opportunity service
- Update README and tests to reflect spot-only architecture
- Bump version to 2.0.7

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/coinhunter/config.py

@@ -21,7 +21,6 @@ output_format = "tui"
 
 [binance]
 spot_base_url = "https://api.binance.com"
-futures_base_url = "https://fapi.binance.com"
 recv_window = 5000
 
 [market]
@@ -31,7 +30,6 @@ universe_denylist = []
 
 [trading]
 spot_enabled = true
-futures_enabled = true
 dry_run_default = false
 dust_usdt_threshold = 10.0