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/toml/src/fmt.rs

@@ -0,0 +1,66 @@
+#[derive(Copy, Clone, Default)]
+pub(crate) struct DocumentFormatter {
+    pub(crate) multiline_array: bool,
+    is_value: bool,
+}
+
+impl toml_edit::visit_mut::VisitMut for DocumentFormatter {
+    fn visit_document_mut(&mut self, node: &mut toml_edit::DocumentMut) {
+        toml_edit::visit_mut::visit_document_mut(self, node);
+    }
+
+    fn visit_item_mut(&mut self, node: &mut toml_edit::Item) {
+        let is_parent_value = self.is_value;
+        if !is_parent_value {
+            let other = std::mem::take(node);
+            let other = match other.into_table().map(toml_edit::Item::Table) {
+                Ok(i) => i,
+                Err(i) => i,
+            };
+            let other = match other
+                .into_array_of_tables()
+                .map(toml_edit::Item::ArrayOfTables)
+            {
+                Ok(i) => i,
+                Err(i) => i,
+            };
+            self.is_value = other.is_value();
+            *node = other;
+        }
+
+        toml_edit::visit_mut::visit_item_mut(self, node);
+        self.is_value = is_parent_value;
+    }
+
+    fn visit_table_mut(&mut self, node: &mut toml_edit::Table) {
+        node.decor_mut().clear();
+
+        // Empty tables could be semantically meaningful, so make sure they are not implicit
+        if !node.is_empty() {
+            node.set_implicit(true);
+        }
+
+        toml_edit::visit_mut::visit_table_mut(self, node);
+    }
+
+    fn visit_value_mut(&mut self, node: &mut toml_edit::Value) {
+        node.decor_mut().clear();
+
+        toml_edit::visit_mut::visit_value_mut(self, node);
+    }
+
+    fn visit_array_mut(&mut self, node: &mut toml_edit::Array) {
+        toml_edit::visit_mut::visit_array_mut(self, node);
+
+        if !self.multiline_array || (0..=1).contains(&node.len()) {
+            node.set_trailing("");
+            node.set_trailing_comma(false);
+        } else {
+            for item in node.iter_mut() {
+                item.decor_mut().set_prefix("\n    ");
+            }
+            node.set_trailing("\n");
+            node.set_trailing_comma(true);
+        }
+    }
+}