Studio Images: add the FLUX.2-dev model family

Loading unsloth/FLUX.2-dev-GGUF failed because detect_family knew only the
Qwen3-based FLUX.2-klein, so FLUX.2-dev (the full, Mistral-based Flux2Pipeline)
resolved to nothing and the load errored. Add a flux.2-dev family: Flux2Pipeline
+ Flux2Transformer2DModel over the black-forest-labs/FLUX.2-dev base repo (gated,
reachable with an HF token), with its FLUX.2 32-channel VAE and Mistral text
encoder wired for the sd-cli path from the open Comfy-Org/flux2-dev mirror.
text-to-image only: diffusers 0.38 ships no Flux2 img2img / inpaint pipeline for
dev. Frontend gets sensible dev defaults (28 steps, guidance 4), distinct from
klein's turbo defaults. Verified live: GGUF load resolves the family + gated base
repo and generates a real 1024x1024 image on GPU.

studio/frontend/src/features/images/images-page.tsx

@@ -194,6 +194,8 @@ const MODEL_DEFAULTS: Array<{ match: string; steps: number; guidance: number }>
   { match: "kontext", steps: 28, guidance: 2.5 },
   { match: "flux.1", steps: 28, guidance: 3.5 },
   { match: "flux.2-klein", steps: 4, guidance: 0 },
+  // FLUX.2-dev is the full (non-distilled) model: more steps + real guidance, unlike klein.
+  { match: "flux.2-dev", steps: 28, guidance: 4 },
   { match: "qwen-image", steps: 20, guidance: 4 },
   { match: "z-image", steps: 20, guidance: 4 },
 ];