feat(web): UI controls for EV loadpoint target SoC + deadline #175

Why

Field test by @erikarenhill surfaced that without a user-set `target_soc_pct` + `target_time` on a loadpoint, the planner has no urgency penalty for the EV and rationally defers charging — especially in arbitrage mode, where idle PV can be exported for revenue instead of going into the car.

Backend is fully wired:

• `POST /api/loadpoints/{id}/target` in `go/internal/api/api.go:1758`
• `loadpoint.Manager.SetTarget(id, socPct, targetTime)`
• Triggers an MPC replan on success
• Loadpoint manager persists `targetSoCPct` + `targetTime` fields

What's missing: any UI to call that endpoint. No buttons, forms, or modals. Users have to curl the API to set a target.

What to build

On the loadpoint card / EV charging view:

• Target SoC input (integer %, clamped 0–100)
• Target time input (date + time picker, "by when")
• Save button that POSTs to `/api/loadpoints/{id}/target`
• Display current target (SoC % + time remaining) when set
• "Clear target" button so users can drop the deadline

Optional nice-to-haves:

• Quick presets (80 % by 07:00, 100 % by 18:00, etc.)
• Visual indicator when the current plan can't hit the target (the DP has a "shortfall" concept — see `mpc.go:445–450`)

Knock-on effect

This unblocks EV charging in arbitrage mode. Erik reported arbitrage "doesn't work" for EV — it's not broken, it just has no incentive to charge without a deadline. With a target set, the DP gets a 4× mean-price shortfall penalty and commits.

Reported by

@erikarenhill (Discord field test):

Nästa finding, posta SoC target för EV Charging saknar ju helt GUI eller automatik — därför den också slår av