meta-issue: mixed currency

[aaronspring]

#396 and #400 is about salary confusion. more to come

proposal: only use one base currency for dashboard, tax, salary. convert invoiced amounts to base currency based on month mean conversion

[clstaudt]

Mixed-currency business has so far not been in scope. The simplifying assumption was that the freelancer operates in one locale with one currency. Happy for suggestions on how to handle this consistently.

[aaronspring]

Proposal: set a primary currency and (when checkbox enableď) automatically convert into that currency using monthly average conversation rate from some official source) minus 1% conversion (editable) costs.
Does it need to be 100% correct? Yes if used for taxes, no if just used for salary to spend

[aaronspring]

see #419