this pull request contains some small ui improvements and bugfixes.

fix:

• validate amount on payment page.
  a value of zero will actually send the entire amount.
  this is probably not what the user expects.
  solving this by requiring the value to be greater than zero.
• correctly display qr code on receive page.
  when navigating to the receive page with a specific account,
  the qr code has not been loaded. this fix will ensure the account
  type is an integer and hence the qrcode is loaded as expected.

ui:

• use same width for value/account form elements
• select account via select box on payment page
• select account via select box on receive page