fix(widgets): forward portable-atomic feature to ratatui-widgets - #2822

Carries over #2771, which I closed by mistake.

What

The portable-atomic feature on ratatui now also forwards to ratatui-widgets, so builds for targets without native atomic pointers (for example thumbv6m-none-eabi on the RP2040) work with the documented setup.

Root cause

ratatui's portable-atomic feature only forwarded to ratatui-core. ratatui-widgets still imported alloc::sync::Arc unconditionally in block/shadow.rs, and alloc::sync only exists when the target supports atomic pointers. A cargo check --target thumbv6m-none-eabi against a crate enabling ratatui/portable-atomic fails with:

error[E0432]: unresolved import `alloc::sync`
  --> ratatui-widgets/src/block/shadow.rs:1:12
   |
 1 | use alloc::sync::Arc;
   |            ^^^^ could not find `sync` in `alloc`

The change

• ratatui-widgets gains a portable-atomic feature backed by an optional portable-atomic-util dependency, mirroring how ratatui-core already integrates portable-atomic through kasuari.
• block/shadow.rs cfg-selects the Arc import: portable_atomic_util::Arc when the feature is on, alloc::sync::Arc otherwise.
• portable-atomic's Arc does not implement CoerceUnsized on stable Rust, so with the feature on, Shadow::custom erases the concrete effect type through a Box before converting to Arc<dyn CellEffect>. Without it, the code keeps the plain Arc::new. This keeps the same public API and Clone behavior on both paths.
• The portable-atomic feature on ratatui now forwards to ratatui-widgets/portable-atomic in addition to ratatui-core/portable-atomic.

Verification

• A no_std scratch crate depending on this branch with ratatui = { default-features = false, features = ["portable-atomic"] } plus portable-atomic with critical-section (the setup from the book's embedded chapter): cargo check --target thumbv6m-none-eabi fails before this change with the E0432 above and passes after.
• cargo build --target x86_64-unknown-none -p ratatui-widgets (the repository's no-std CI job) passes.
• cargo xtask format --check and the clippy gate pass locally, and cargo test --all-features -p ratatui-widgets passes with no new failures. My local environment is Windows, so the unix-only crates (termion backend) could not be compiled locally.

This PR was prepared with AI coding assistance and reviewed line by line.

Fixes #2767