Recommend “Always-on VPN” (without blocking) to improve reliability

### Issue Details

On some devices (e.g. Surface Duo, Samsung, foldables), AdGuard’s local VPN is significantly more reliable when “Always-on VPN” is enabled.

It helps prevent the VPN service being stopped by battery optimisation and improves reliable restart after system power-on or reboot.

This is not about “Block connections without VPN” (which should not be enabled). This request is Always-on VPN only.

Why
- Some OEMs aggressively stop background VPN services
- Battery optimisation and boot timing can interrupt AdGuard
- Always-on VPN tells Android to automatically restart the VPN
- It does not change routing, DNS, encryption, or block traffic

Summary
- Safe when not combined with blocking
- Mitigates battery optimisation kills
- Improves post-reboot startup reliability
- Improves real-world stability

### Proposed solution

Mirror existing battery optimisation guidance:
- Conditional recommendation only
- OEM allow/deny list
- One-time, dismissible message
- Deep link to VPN settings

Example:
“Improve AdGuard reliability
Enabling Always-on VPN may improve stability on your device.
(Do not enable ‘Block connections without VPN’.)”

### Alternative solution

_No response_