[Stable Diffusion] Add the three main performance options as exposed toggles

[NineBallo]

This adds the three main performance toggles to the webui.
--diffusion-fa, --diffusion-conv-direct, and --vae-conv-direct

---- AI Description
It respects that vulkan & cuda has fa forced and locks that one when they are set.

Add three new boolean configuration options for the sd-cpp backend that control GPU compute features:

• Diffusion FA (--diffusion-fa) — Enable fused-attention
• Diffusion Conv Direct (--diffusion-conv-direct) — Enable direct convolution
• VAE Conv Direct (--vae-conv-direct) — Enable VAE direct convolution

Key Features:

1. Generic backend descriptor — Options are defined in sdcpp.h descriptor. New boolean options automatically become CLI flags without code changes in the server merge function.

2. Backend-forced flags — Vulkan and CUDA backends require --diffusion-fa. The server enforces this: if the user unchecks the box, the flag still gets added to sdcpp_args.

3. UI disabled checkboxes — When a backend forces an option (e.g., Vulkan forces Diffusion FA), the checkbox is disabled with a 🔒 lock icon and tooltip explaining "This option is required by the
   selected backend".

4. recipe_options API — The new options are exposed in GET /api/v1/models//recipe_options so the frontend can check force_* fields.

Files Changed:

• src/cpp/include/lemon/backends/sdcpp/sdcpp.h — Add options to descriptor
• src/cpp/server/server.cpp — Generic build_sdcpp_args_merge() function with forced-flag logic
• src/app/src/renderer/ModelOptionsModal.tsx — Disabled checkbox UI with lock icon
• src/app/src/renderer/recipes/recipeOptionsConfig.ts — Frontend option mappings
• src/app/styles/styles.css — Disabled checkbox + locked indicator styles

[jeremyfowers]

@NineBallo please be aware that we have a feature freeze on gui2, and the gui changes will need to go into GUI3. If you can remove the gui features from this PR and rebase them into gui3 in a future PR that would be great.

[jeremyfowers]

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