frontend: store: Divide disk write rate by linux2rest sample window

[joaoantoniocardoso]

Summary

• The tray disk widget (W: … MB/s) treated each available_space_B jump as happening over the 2 s frontend poll. linux2rest only refreshes that snapshot every 5 s, so a ~2.1 MB/s recording showed as ~5.0–5.1 MB/s, then stuck there because a later zero delta kept the inflated rate.
• Divide capacity deltas by the same 5 s Sampler window already used for network (linux2restProbeRateBps). Match disks by mount_point (many bind mounts share name: /dev/root). Drop unused Disk.last_update.
• No linux2rest API change. Self-hosted frontend against a vehicle writing ~2 MB/s showed W: 2.0 MB/s.

Helps #4172: this PR doesn't solve it, but at least we get a correct reading from the widget
Related (not required for this PR): patrickelectric/linux2rest#38

Test plan

• Self-host core/frontend with BLUEOS_ADDRESS=http://<vehicle>/ yarn dev
• With a sustained ~16 Mbps recording (or any ~2 MB/s writer), tray shows ~2 MB/s, not ~5 MB/s, and holds there across several 2 s polls
• After stopping the writer, rate drops toward zero on the next non-zero capacity change
• Disk fill still updates (xx/yyGB (zz%))
• Network tray rates still look sane (this PR reuses their helper, does not change that path)

[patrickelectric]

Because 1 approve is not enough