[Web-Ext][Bug] Cadence incorrect for walking activities, at least in activities tab

[bartvanandel]

Describe the bug
Activities nicely show cadence for both cycling and walking (running, hiking, ...) activities. For cycling they look fine to me, but for walking, they are off by a factor of 2. I guess 2 steps are counted as 1 RPM, which for walking doesn't make much sense. You'd never ask someone "count 10 paces" and actually mean 20 footsteps. So, maybe it's better to display SPM (steps per minute) for walking type activities.

To Reproduce
Steps to reproduce the behavior:

1. Go to 'Activities' tab
2. Find a walking activity
3. Look at the value in the 'Average cadence' column
4. Conclude that the value is quite low

Expected behavior
Value should be twice what is currently displayed (e.g. 110 instead of 55), and use "SPM" instead of "RPM".

Environment

• OS: MacOS, but likely irrelevant. If I'd have to guess, I think the app probably works the same.
• Browser name and version: Chrome 130, but likely irrelevant
• Web extension version: 7.2.0 / Build 20240811.dd77acf4