Refactor Spacing into DynamicSpacing using proc macro (#20504)

Density tracking issue: #18078 

This PR refactors our spacing system to use a more flexible and
maintainable approach. We've replaced the static `Spacing` enum with a
dynamically generated `DynamicSpacing` enum using a proc macro.

Enum variants now use a `BaseXX` format, where XX = the pixel value @
default rem size and the default UI density.

For example:

`CustomSpacing::Base16` would return 16px at the default UI scale &
density.

I'd love to find another name other than `Base` that is clear (to avoid
base_10, etc confusion), let me know if you have any ideas!

Changes:

- Introduced a new `derive_dynamic_spacing` proc macro to generate the
`DynamicSpacing` enum
- Updated all usages of `Spacing` to use the new `DynamicSpacing`
- Removed the `custom_spacing` function, mapping previous usages to
appropriate `DynamicSpacing` variants
- Improved documentation and type safety for spacing values

New usage example:

```rust
.child(
    div()
        .flex()
        .flex_none()
        .m(DynamicSpacing::Base04.px(cx))
        .size(DynamicSpacing::Base16.rems(cx))
        .children(icon),
)
```

vs old usage example:

```
.child(
    div()
        .flex()
        .flex_none()
        .m(Spacing::Small.px(cx))
        .size(custom_spacing(px(16.)))
        .children(icon),
)
```

Release Notes:

- N/A

crates/ui_macros/src/ui_macros.rs

@@ -1,4 +1,5 @@
 mod derive_path_str;
+mod dynamic_spacing;
 
 use proc_macro::TokenStream;
 
@@ -51,3 +52,9 @@ pub fn path_str(_args: TokenStream, input: TokenStream) -> TokenStream {
     // This attribute doesn't modify the input, it's just a marker
     input
 }
+
+/// Generates the DynamicSpacing enum used for density-aware spacing in the UI.
+#[proc_macro]
+pub fn derive_dynamic_spacing(input: TokenStream) -> TokenStream {
+    dynamic_spacing::derive_spacing(input)
+}