v1.16.6 - Dashboard maintenance moved to HA storage

Fixes the editor bugs from v1.16.5 (focus loss on every keystroke, items vanishing on re-edit) by moving the maintenance list out of the card config into the integration's HA storage.

What changed

• Maintenance items are now stored per bike in HA storage (/config/.storage/) instead of the browser's localStorage. The list syncs across all devices, survives browser changes, and is the same data the integration's existing Maintenance Items Due sensor reads.
• Editor: bike picker dropdown at the top, maintenance items load from the backend and save live (400ms debounce per field) via WebSocket. No more Lovelace round-trip per keystroke - the focus loss is gone, and re-opening the dashboard shows the items correctly.
• WebSocket API: new commands bosch_ebike/add_maintenance, /update_maintenance, /complete_maintenance, /remove_maintenance, exposed alongside the existing /list_maintenance.
• New service: bosch_ebike.update_maintenance (in addition to the existing add/complete/remove services) so automations can edit existing items.

Migration

When the dashboard card first renders with a bike selected, it imports any legacy maintenance: entries from the card config into the backend (idempotent: only runs if the backend list for that bike is empty). LocalStorage 'done' markers from v1.16.5 get cleaned up at the same time.

i18n updated for EN / DE / NL, README sections rewritten.