feat(studio): WiFi token memory

[pbertsch]

Summary

Studio now remembers the agent token per discovered WiFi device. After a successful `ConnectWiFi`, subsequent mDNS-discovered sessions for that device prefill the token automatically — eliminating the only friction point in the v0.7.0 mDNS flow.

v0.7.1 PR 1 of 3.

How it works

• Token store is a `Record<string, string>` keyed by the Bonjour instance name (e.g. `Patricks-iPhone`), persisted in localStorage as JSON.
• On `wifi:device-found`: if a token is remembered for that name, the device list shows `🔑 saved` plus a `✕` forget button.
• On device click: the token input is prefilled (still editable for cases where the device's token rotated).
• On successful connect: the token (whether prefilled or freshly entered) is saved.
• On `✕` click: removes the entry and toasts a confirmation.

Trust boundary

Tokens are persisted in the WebView's localStorage, which is sandboxed inside the Studio desktop app — same trust boundary as the user's filesystem. The agent token gates an LAN-only test surface (not production credentials), so this is an acceptable storage choice for v0.7.x. If we ever need real secret storage we'll move to macOS Keychain via the `keyring` Go package.

Keyed by name (not host:port) because IPs change across DHCP leases but device hostnames are stable.

Test plan

• `tsc --noEmit` clean
• Manual: first connect — open WiFi modal, click discovered device, paste token, connect. Verify subsequent reopen of modal shows `🔑 saved` for that device.
• Manual: prefill — close + reopen modal, click the same device, verify token input is prefilled.
• Manual: forget — click `✕` next to a remembered device, verify the saved tag and forget button disappear, verify token input is empty on next click.