fix: full arb table + net profit columns in price view

- ArbTable: removed slice(0,10) cap, shows all 42 opps (21x2)
- PriceTable: added net_bg_to_hl and net_hl_to_bg columns
- Backend SSE: sends net profit for both directions per coin

check_db.py

@@ -0,0 +1,6 @@
+import sqlite3
+conn = sqlite3.connect('data/trades.db')
+rows = conn.execute('SELECT id, scale_count, amount_usd, fee_entry, fee_exit, net_pnl FROM trades').fetchall()
+for r in rows:
+    print(f'  #{r[0]}: scales={r[1]} amt=${r[2]} fee_entry={r[3]:.6f} fee_exit={r[4]:.6f} net={r[5]:.6f}')
+conn.close()