fix: link CoreGraphics framework for Metal device detection

[aupadhyay]

Summary

cake chat panics on a fresh macOS clone with --features metal:

swap_remove index (is 0) should be < len (is 0)
stack backtrace:
   0: __rustc::rust_begin_unwind
   1: core::panicking::panic_fmt
   2: alloc::vec::Vec<T,A>::swap_remove::assert_failed
   3: candle_core::metal_backend::<impl candle_core::backend::BackendDevice for candle_core::metal_backend::device::MetalDevice>::new
   4: cake_core::utils::get_inference_device
   5: cake_core::cake::Context::from_args
   6: cake::main::{{closure}}
   7: tokio::runtime::runtime::Runtime::block_on
   8: cake::main
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
zsh: abort      RUST_BACKTRACE=1 ./target/release/cake chat Qwen/Qwen3-0.6B

Fix

• The call chain leads us to candle_core and candle-metal-kernels lines 50-55 which calls MTLCreateSystemDefaultDevice() and collects into a Vec.

• MTLCreateSystemDefaultDevice() returns None because CoreGraphics is not linked. Documented requirement in objc2-metal (see objc2-metal/src/lib.rs lines 14-16):

  NOTE: To use MTLCreateSystemDefaultDevice you need to link to CoreGraphics

• candle-metal-kernels doesn't add this linkage, and neither does cake

• One line fix in cake-core/build.rs

• Verified chat works on metal after fix