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all is said in the title...
a web app if existing would be enough as far as possible to save the finish drawing as well as a file to continue later if needing!
(info for those using OO and/or derivates: the module «draw» offers VERY GREAT possibilities but I use myself the gnome office 
(draw can read and change autoCAD exchange files!)
@oui
I have been playing with https://www.figuro.io/Designer It appears that it works great in Firefox. I am not sure what file-types you can export though. I have been designing a hexagonal building consisting of 3 hexagonal prisms. Now I need to find some simple architectural software to do the final layout.
Puppy Linux. It just works!
Foassapup 9.5 64-(Frugal) Mobo, M5A99FX PRO - CPU, AMD FX-6300 6 core - mem,32GB - Storage
ssd 128GB M4 - ssd-256GB APS - ssd-1TB PNY - GPU GeForce GTX 1060 GAMING X 6G. Yes it's a Mutt.
I did my first simple CAD sketches with Qcad, years ago.
I see nowadays you have a free "QCAD Community Edition" and a pro version. Not sure if you can install it from the puppy package manager.
FreeCad is a complex and full CAD with a long learning curve, perhaps it doesn't fit your "simple home usage" requirement.
Mikewalsh has made a QCAD portable here:
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