Yes, I did. Saves made during testing your project online in Construct 3 ("Play") are saved based on your browser. For chrome: C:\Users\USERNAME\AppData\Local\PROJECTNAME\User Data\Default\Local Storage\leveldb
For nw.js export the path is always the same, since nw.js uses Chromium (chrome browser sub-engine). It can be located here: C:\Users\USERNAME\AppData\Local\PROJECTNAME\User Data\Default\IndexedDB\chrome-extension_ccnecbdlmggdebaobiodilckckknoohl_0.indexeddb.leveldb
The last part with some unrecognisable pattern of letters, seems to be generated based on your project (name, possibly). To easily get to Appdata simply write %appdata% in your windows explorer (since appdata is usually hidden for users on most pc's).
The main file that is being save is called "00003.log", all the data you're trying to save via Local Storage is being saved there. One problem is - each time you save - this 00003 file will get a bit bigger. That's because Local Storage does not seem to rewrite saved data - it is adding new lines in the document and Construct 3 uses the last "save" (hence - the file is getting new information and is getting bigger each time the save is done; in my example - about 60 kilobytes of data is added each time).
Another important thing is: after "00003.log" get's to around 1Mb, it creates "00004.log" in the same folder and starts to write there instead (same with 00005, 00006, etc.). Each file contains a a lot of saves though (about 70 in my example) so I would not worry too much about the size of the file, unless your game saves like every 1 minute.
So instead of saving just 00003.log, I recommend to save the whole folder containing it (and other 0000X+.log files) in Steam Cloud by using the "*" instead of the exact file name to save. Worked for me.
PS: Oh, yeah. And this save location is the same for your demo-version, so it's best to sync demo-version and full-game saves in Steam if you want the players that tried out the demo version to continue playing from the saved spot in the full game.