The global "secondary card VRAM cap" was a single number applied to every
non-lead card. Replace it with a per-CARD map so each physical card on the
machine can be capped independently — e.g. RX 580 → 4 GB, RTX 3090 → 20 GB.

- gpu_detect: card_key() (nvidia:<uuid> / amd:<pci>) + gpu_cards() enumerating
  every physical GPU. Stable keys the front and engines both compute identically.
- vulkan: _per_device_card_key() (parallel to _per_device_free_vram_gb, same
  llama.cpp order) + _resolve_device_caps() combining the per-model secondary cap
  (non-main devices) with the global per-card map, lowest wins per device. Applied
  in both the auto-offload pool and the auto-split ratio.
- config: OffloadConfig.split_card_caps_gb {key: gb}; per-model
  split_secondary_cap_gb stays a single value (one cap is enough per model).
- manager: pass _global_card_caps to the engine; keep per-model scalar.
- settings UI: render one cap input per detected card (name + vendor), keyed by
  the stable card key; GET returns gpu_cards + saved caps, POST saves the map.

Applies on the next engine (re)load (global args are read at engine startup),
matching the previous global cap's behavior.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011DDv7BchtZQWsnPG6Jm49m