Improve menu items for clarity, reduce copy-pasted Strings and bugfix fiat conversion

[ThomasFarstrike]

• Fix fiat conversion API call compatibility with LNBits ("from" changed to "from")
• Change "LNPoS", "Offline PoS", "OnChain" and "ATM" menu items to more explicit "Receive Online", "Receive Offline", "Receive OnChain" and "Send Offline" (more detailed reasoning in separate commit messages)
• Move strings that are used more than once to a #define to avoid copy-paste, because this often causes bugs/inconsistencies due to changing one, but not the other(s).

[ThomasFarstrike]

I've tested all the changes on the actual hardware, including all the menu entries.