WiFi: /wifi web endpoint to list, add, delete, reorder, forget-all and handle the SD file (#99 child 2)

[Strycher]

Epic: #99 · Parent: #99 — child 2 of 4. Plan: docs/plans/2026-09-09-wifi-configuration.md

• List saved networks (SSID, priority, never the password), add, delete, move up. POST forms in the style of /geocaches. Plain HTTP on the LAN, same trust model as every endpoint.
• Forget all networks: two-step confirm before the store is wiped (owner, 2026-09-09).
• While /config/wifi.txt is present on the SD card: show it and offer Remove or Keep; nothing happens to the file without that choice (owner, 2026-09-09).

Acceptance: every action verified from a browser on the bench; both envs build.