Generate iOS SPIR-V headers without building the application

[Copilot]

The iOS workflow failed during configuration because required pre-generated SPIR-V headers were missing. Header generation was incorrectly coupled to the full application target, which also depends on private assets unavailable in CI.

• CMake: Adds UnleashedRecompShaders, aggregating shader-generation outputs from existing custom commands.
• Workflow: Builds the dedicated shader target before configuring the iOS build, with generation errors no longer suppressed.

get_property(UNLEASHED_RECOMP_SHADER_OUTPUTS
    GLOBAL PROPERTY UNLEASHED_RECOMP_SHADER_OUTPUTS)
add_custom_target(UnleashedRecompShaders
    DEPENDS ${UNLEASHED_RECOMP_SHADER_OUTPUTS})