Fix Buildroot setup hotspot startup

[mariusandra]

Summary

Fixes the standalone Buildroot setup image path so flashed Raspberry Pi images actually start the FrameOS runtime and therefore bring up the documented FrameOS-Setup access point.

Root Cause

The release image configuration requested wifiHotspot: bootOnly with SSID FrameOS-Setup, but the root filesystem did not install or enable frameos.service. The only existing firstboot setup unit was gated on /boot/frameos-setup.json, which is absent from the standalone release image, so nothing started the runtime that owns hotspot setup.

Changes

• Install and enable frameos.service in Buildroot root filesystems.
• Patch composed/precompiled SD images so cached release bases also receive the service and hostname.
• Include the service in newly generated Buildroot base overlays.
• Update the setup confirmation page to tell users where to find the frame after reconnecting, including the admin URL when enabled.
• Add backend and Nim portal tests for the new behavior.

Validation

• Patched a throwaway copy of the exact v2026.6.27 Buildroot release image and confirmed frameos.service, its systemd wants symlink, and /etc/hostname are present.
• ./.venv/bin/python3 -m pytest -q backend/app/tasks/tests/test_buildroot_release_image.py backend/app/tasks/tests/test_buildroot_image.py
• nim c -r src/frameos/tests/test_portal.nim
• git diff --check