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/embassy-executor-macros/src/util.rs

@@ -0,0 +1,74 @@
+use std::fmt::Display;
+
+use proc_macro2::{TokenStream, TokenTree};
+use quote::{ToTokens, TokenStreamExt};
+use syn::parse::{Parse, ParseStream};
+use syn::{braced, bracketed, token, AttrStyle, Attribute, Signature, Token, Visibility};
+
+pub fn token_stream_with_error(mut tokens: TokenStream, error: syn::Error) -> TokenStream {
+    tokens.extend(error.into_compile_error());
+    tokens
+}
+
+pub fn error<A: ToTokens, T: Display>(s: &mut TokenStream, obj: A, msg: T) {
+    s.extend(syn::Error::new_spanned(obj.into_token_stream(), msg).into_compile_error())
+}
+
+/// Function signature and body.
+///
+/// Same as `syn`'s `ItemFn` except we keep the body as a TokenStream instead of
+/// parsing it. This makes the macro not error if there's a syntax error in the body,
+/// which helps IDE autocomplete work better.
+#[derive(Debug, Clone)]
+pub struct ItemFn {
+    pub attrs: Vec<Attribute>,
+    pub vis: Visibility,
+    pub sig: Signature,
+    pub brace_token: token::Brace,
+    pub body: TokenStream,
+}
+
+impl Parse for ItemFn {
+    fn parse(input: ParseStream) -> syn::Result<Self> {
+        let mut attrs = input.call(Attribute::parse_outer)?;
+        let vis: Visibility = input.parse()?;
+        let sig: Signature = input.parse()?;
+
+        let content;
+        let brace_token = braced!(content in input);
+        while content.peek(Token![#]) && content.peek2(Token![!]) {
+            let content2;
+            attrs.push(Attribute {
+                pound_token: content.parse()?,
+                style: AttrStyle::Inner(content.parse()?),
+                bracket_token: bracketed!(content2 in content),
+                meta: content2.parse()?,
+            });
+        }
+
+        let mut body = Vec::new();
+        while !content.is_empty() {
+            body.push(content.parse::<TokenTree>()?);
+        }
+        let body = body.into_iter().collect();
+
+        Ok(ItemFn {
+            attrs,
+            vis,
+            sig,
+            brace_token,
+            body,
+        })
+    }
+}
+
+impl ToTokens for ItemFn {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        tokens.append_all(self.attrs.iter().filter(|a| matches!(a.style, AttrStyle::Outer)));
+        self.vis.to_tokens(tokens);
+        self.sig.to_tokens(tokens);
+        self.brace_token.surround(tokens, |tokens| {
+            tokens.append_all(self.body.clone());
+        });
+    }
+}