Show tracked tokens with zero balance on main and address pages

Add showZeroBalanceTokens setting (default: on). When enabled,
balanceLinesForAddress merges state.trackedTokens with the address's
tokenBalances, showing 0.0000 lines for tracked tokens that have no
balance on that address. This gives users visibility into all tokens
they're watching across all addresses.

src/popup/index.html

@@ -502,6 +502,19 @@
                     </button>
                 </div>
 
+                <div class="bg-well p-3 mx-1 mb-3">
+                    <h3 class="font-bold mb-1">Display</h3>
+                    <label
+                        class="text-xs flex items-center gap-1 cursor-pointer"
+                    >
+                        <input
+                            type="checkbox"
+                            id="settings-show-zero-balances"
+                        />
+                        Show tracked tokens with zero balance
+                    </label>
+                </div>
+
                 <div class="bg-well p-3 mx-1 mb-3">
                     <h3 class="font-bold mb-1">Ethereum RPC</h3>
                     <p class="text-xs text-muted mb-1">