Add address-token detail view for per-token transaction filtering

Clicking a token balance on the address detail view navigates to a
focused view showing only that token's transactions. Send pre-selects
and locks the token dropdown, Receive shows an ERC-20 warning for
non-ETH tokens, and all back buttons return to the correct parent view.

src/shared/state.js

@@ -31,6 +31,7 @@ const state = {
     ...DEFAULT_STATE,
     selectedWallet: null,
     selectedAddress: null,
+    selectedToken: null,
 };
 
 async function saveState() {