fix(wifi): add reconnect watchdog for brcmfmac roam/SAE wedge

[fabiiw05]

Problem

Power save (fixed in #8) was not the only WiFi drop. On a WPA2/WPA3-transition, band-steering AP (one SSID, several BSSIDs across 2.4/5 GHz), NetworkManager's background scan (bgscan simple:30:-65:300) roams to another BSSID and brcmfmac fails SAE external auth on the transition BSSID:

kernel: brcmf_cfg80211_external_auth: External authentication failed: status=1

The link then dies — or stays connected while passing no traffic — until a manual nmcli reconnect. Verified on hardware; power_save was already off, so this is a distinct bug from #8.

Fix

WiFi recovery watchdog (scripts/customize.sh):

• /usr/local/sbin/uconsole-wifi-watchdog driven by uconsole-wifi-watchdog.timer (every 30 s).
• If wlan0 is disconnected, or connected but the default gateway is unreachable across two probes, it forces a NetworkManager reconnect.
• Only acts when an autoconnect wifi profile exists (never fights a deliberate disconnect).
• Uses only NetworkManager + iproute2 + iputils (all already in the ALARM base) — no extra package.
• Inspect with journalctl -t uconsole-wifi-watchdog.

Docs (MAINTAINING.md / MAINTAINING.ja.md): restructure the WiFi section into Cause 1 — power save / Cause 2 — failed roam, and document the per-device 5 GHz band-pin mitigation (nmcli con modify <SSID> 802-11-wireless.band a). The band pin is network-specific, so it stays a device tweak, not an image default.

Verification

• Diagnosed on hardware: power_save: off confirmed (fix(wifi): disable brcmfmac power save to prevent occasional drops #8 working), -57 dBm / 5 GHz / 433 Mbit healthy link, single external_auth failed: status=1 correlating with a manual reconnect.
• Watchdog bash -n clean; live dry-run is a clean no-op (exit 0) while connected + gateway reachable; deployed live and running every 30 s (Result=success, no-op = no log, as designed).
• Band-pin applied on the affected device (5 GHz locked, cross-band steering eliminated).

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