Trade and portfolio pages ignore the selected network

[Mosas2000]

Several frontend pages still fetch read-only data from STACKS_MAINNET directly instead of using the active network from the app context.

Evidence:

• frontend/src/pages/TradePage.tsx:4,66-73 imports STACKS_MAINNET and passes it to fetchCallReadOnlyFunction for get-market.
• frontend/src/pages/PortfolioPage.tsx:4,40-47,74-81 does the same for get-user-position.
• The app already exposes network switching through NetworkContext, so these pages bypass the selected network.

Impact:

• Switching to testnet still leaves these pages reading mainnet contract state.
• Trade/portfolio views can show the wrong market data or fail outright when the selected network differs from mainnet.

Reproduction:

1. Switch the app to testnet.
2. Open a market in Trade or Portfolio.
3. Observe that the read-only contract calls still target mainnet.

Priority: High

Suggested scope:
Use the active network from the network context or network utilities for all read-only contract calls in these pages.