fix(gallery): correct meta-backend definitions in backend/index.yaml

Backends that ship per-platform images must be meta backends (a capabilities
map and NO uri) so the right variant is auto-selected per platform - mirroring
llama-cpp/whisper. Several entries were misdefined; fixed here:

- Concrete base + metal sibling (could not select the Apple Silicon variant):
  silero-vad, piper, kitten-tts, local-store (+ their -development). Converted
  each anchor to a meta and added the cpu-<name> concrete.
- mlx family (mlx, mlx-vlm, mlx-audio, mlx-distributed + -development): anchor
  had both a uri AND a capabilities map, so IsMeta() was false and the map was
  ignored (always resolved to the metal-darwin image); the metal-<name> target
  did not exist. Removed the uri and added the missing metal-<name> concretes.
- Dangling capability targets: diffusers/kokoro nvidia-l4t-cuda-12 repointed to
  the existing nvidia-l4t-<name> concrete; coqui nvidia-cuda-13 key removed
  (no cuda13-coqui image).
- locate-anything: the meta existed but its concrete entries were never added,
  so it was un-installable on every platform. Added the full concrete set plus
  the locate-anything-development meta, mirroring rfdetr-cpp. Image tags grounded
  against the published quay.io tags.

Assisted-by: claude:claude-opus-4-8 [Claude Code]
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

Author: mudler