ui : move model memory-fit constants to lib
Move the hardware compatibility budget constants (MIB_BYTES, MB_PER_GB,
QUANT_WEIGHT, RAM_BUDGET_RATIO, RAM_OVERHEAD_MB, MAC_MEM_TIERS) out of
model-compatibility.ts into a new models-scoped constants file, so the
estimation util imports them from $lib/constants.
Assisted-by: llama-ui:Qwen3.8-Flash-Next
