Fix/sad.jl updates

[kandread]

• Updated base Julia image in Dockerfile from 1.11.6 to 1.12.5.
• Updated Sad.jl submodule to the latest version.
• Replaced Sad.drop_unobserved with Sad.preprocess.
• Updated Sad.priors to handle the new consolidated prior object.
• Replaced Sad.estimate with Sad.infer and updated the logic for extracting posterior results.
• Updated Qa and Qu initialization to use Matrix{Sad.FloatM}.
• Refactored group access and slicing for improved readability and compatibility.
• Simplified time_str generation by converting time values directly to strings.
• Added coalesce in write_output to ensure missing values for A0 and n are correctly written as FILL values.