Currently the wallet shows warning triangles on the "Overview" page when the client is not synced.

Looking at the volume of mail I receive from people confused by unsynced client behavior, as well as issues like #7235 this is not enough.

I think it would make sense to:

• Make the warning look more serious. E.g. make the icons red or yellow instead of the same color as the rest.
• The warning should appear on every page, not just Overview but also when receiving or sending transactions. Possibly on the bottom bar.
• Mouseover should warn that:
  • Transactions are not up to date - most notably, received transactions more recent than the current synchronization time are not yet visible. This should be made really, really clear.
  • Balance is not up to date - it does already say this implicitly by showing the triangle next to the balance and recent transactions, but not everyone picks up this hint.
  • Fee estimation is not up to date/incomplete - maybe we need a separate warning for this on the "Send" page.