Move settings-update routing, fallible deserialization recovery,
selection effects, context-menu initialization, picker resize handles,
and ACP client setup into shared implementations. Inline the small
button callback adapters.

`cargo llvm-lines --locked -p settings_ui`:
Before (01c555b435): 1571017 lines, 53634 copies
After: 1177376 lines (-25.1%), 43380 copies (-19.1%)

With compiler wrappers disabled, `cargo clean --target-dir
target/binary-size-split-build` and `cargo build --locked -p zed`:
Before: 135.6s wall, 1569.8s CPU, zed binary 1175563800 bytes 
After: 140.9s wall (+3.9%), 1588.8s CPU (+1.2%), zed binary 1157304600
bytes (-1.6%, -18.3MB)

Release Notes:

- N/A