feat: add display UI with splash screen, dashboard, and RSSI sparkline

Implement full SSD1306 OLED display support:
- Splash screen on boot (1.5s) with project name, version, board
- Dashboard showing radio state, config, counters, RSSI/SNR
- RSSI history sparkline (64-sample ring buffer, 2px-wide bars)
- DisplayOn/DisplayOff host commands routed via dedicated channel
- Pure render functions against generic DrawTarget (no hardware coupling)
- Display on/off control bypasses radio task entirely

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/proc-macro-error-attr/src/lib.rs

@@ -0,0 +1,121 @@
+//! This is `#[proc_macro_error]` attribute to be used with
+//! [`proc-macro-error`](https://docs.rs/proc-macro-error/). There you go.
+
+extern crate proc_macro;
+
+use crate::parse::parse_input;
+use crate::parse::Attribute;
+use proc_macro::TokenStream;
+use proc_macro2::{Literal, Span, TokenStream as TokenStream2, TokenTree};
+use quote::{quote, quote_spanned};
+
+use crate::settings::{Setting::*, *};
+
+mod parse;
+mod settings;
+
+type Result<T> = std::result::Result<T, Error>;
+
+struct Error {
+    span: Span,
+    message: String,
+}
+
+impl Error {
+    fn new(span: Span, message: String) -> Self {
+        Error { span, message }
+    }
+
+    fn into_compile_error(self) -> TokenStream2 {
+        let mut message = Literal::string(&self.message);
+        message.set_span(self.span);
+        quote_spanned!(self.span=> compile_error!{#message})
+    }
+}
+
+#[proc_macro_attribute]
+pub fn proc_macro_error(attr: TokenStream, input: TokenStream) -> TokenStream {
+    match impl_proc_macro_error(attr.into(), input.clone().into()) {
+        Ok(ts) => ts,
+        Err(e) => {
+            let error = e.into_compile_error();
+            let input = TokenStream2::from(input);
+
+            quote!(#input #error).into()
+        }
+    }
+}
+
+fn impl_proc_macro_error(attr: TokenStream2, input: TokenStream2) -> Result<TokenStream> {
+    let (attrs, signature, body) = parse_input(input)?;
+    let mut settings = parse_settings(attr)?;
+
+    let is_proc_macro = is_proc_macro(&attrs);
+    if is_proc_macro {
+        settings.set(AssertUnwindSafe);
+    }
+
+    if detect_proc_macro_hack(&attrs) {
+        settings.set(ProcMacroHack);
+    }
+
+    if settings.is_set(ProcMacroHack) {
+        settings.set(AllowNotMacro);
+    }
+
+    if !(settings.is_set(AllowNotMacro) || is_proc_macro) {
+        return Err(Error::new(
+            Span::call_site(),
+            "#[proc_macro_error] attribute can be used only with procedural macros\n\n  \
+            = hint: if you are really sure that #[proc_macro_error] should be applied \
+            to this exact function, use #[proc_macro_error(allow_not_macro)]\n"
+                .into(),
+        ));
+    }
+
+    let body = gen_body(body, settings);
+
+    let res = quote! {
+        #(#attrs)*
+        #(#signature)*
+        { #body }
+    };
+    Ok(res.into())
+}
+
+#[cfg(not(always_assert_unwind))]
+fn gen_body(block: TokenTree, settings: Settings) -> proc_macro2::TokenStream {
+    let is_proc_macro_hack = settings.is_set(ProcMacroHack);
+    let closure = if settings.is_set(AssertUnwindSafe) {
+        quote!(::std::panic::AssertUnwindSafe(|| #block ))
+    } else {
+        quote!(|| #block)
+    };
+
+    quote!( ::proc_macro_error::entry_point(#closure, #is_proc_macro_hack) )
+}
+
+// FIXME:
+// proc_macro::TokenStream does not implement UnwindSafe until 1.37.0.
+// Considering this is the closure's return type the unwind safety check would fail
+// for virtually every closure possible, the check is meaningless.
+#[cfg(always_assert_unwind)]
+fn gen_body(block: TokenTree, settings: Settings) -> proc_macro2::TokenStream {
+    let is_proc_macro_hack = settings.is_set(ProcMacroHack);
+    let closure = quote!(::std::panic::AssertUnwindSafe(|| #block ));
+    quote!( ::proc_macro_error::entry_point(#closure, #is_proc_macro_hack) )
+}
+
+fn detect_proc_macro_hack(attrs: &[Attribute]) -> bool {
+    attrs
+        .iter()
+        .any(|attr| attr.path_is_ident("proc_macro_hack"))
+}
+
+fn is_proc_macro(attrs: &[Attribute]) -> bool {
+    attrs.iter().any(|attr| {
+        attr.path_is_ident("proc_macro")
+            || attr.path_is_ident("proc_macro_derive")
+            || attr.path_is_ident("proc_macro_attribute")
+    })
+}