WiFi: allow connecting to open networks (no password)

Problem

The appliance rejects WiFi connections to open (password-less) networks. Three layers enforce a password requirement:

1. API (api.rs:3905) — password.len() < 8 check rejects empty passwords
2. Shell script (wifi-connect.sh:36) — exits on empty PSK
3. NM keyfile (wifi-connect.sh:59-62) — always writes key-mgmt=wpa-psk

Fix

• WifiReq.password is now #[serde(default)] (optional, defaults to "")
• API allows empty password (skip the 8-char minimum check)
• Shell script only requires non-empty SSID; PSK can be empty
• NM keyfile: omits [wifi-security] section entirely for open networks
• UI placeholder updated to indicate open networks are supported

Files changed

• src/cognitum-agent/src/api.rs — serde default, relaxed validation, UI placeholder
• scripts/cognitum/wifi-connect.sh — conditional NM keyfile, safe JSON parsing

Test plan

• Connect to WPA2 network (existing behavior, should still work)
• Connect to open network via UI (leave password blank)
• Connect to open network via API: POST /api/v1/wifi/connect {"ssid":"OpenNet"}
• Verify NM keyfile has no [wifi-security] section for open networks

Start with src/cognitum-agent/src/api.rs around api.rs:3905, then inspect scripts/cognitum/wifi-connect.sh at lines 36 and 59-62. Check the existing WPA2 path before testing the listed UI and API open-network cases; done means WPA2 still works and open-network keyfiles omit [wifi-security].