Small install-dialog fixes and improv-wifi-serial-sdk 2.8.1

[kbx81]

What

Four small fixes split out of #721, per review feedback there:

• Bump improv-wifi-serial-sdk to 2.8.1, which clears a stale nextUrl on the client so "Visit Device" can't point at a URL from a previous provisioning session, and adds an optional timeout to requestCurrentState.
• Clear the Improv client's error when entering provisioning. The client never resets it on its own (only new error packets and local RPC timeouts write it), so e.g. a Wi-Fi scan interrupted by a device reset latches a "Timeout" that then renders on the credentials form forever, surviving navigation out of and back into the page. Reproduced on hardware: enter Configure Wi-Fi, hold the device's reset button for a moment — 30s later (the SDK's scan timeout) the red "Timeout" appears and never leaves.
• Guard the scan grace-period timer so a late tick can't populate the network form state after the user has already left the provisioning page.
• Reword the install progress hint ("Keep this page visible for fastest installation.").

[kbx81]

Thanks! 🍻