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/basic-toml/tests/formatting.rs

@@ -0,0 +1,53 @@
+use basic_toml::to_string;
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Clone, Hash, PartialEq, Eq, Serialize, Deserialize)]
+struct User {
+    pub name: String,
+    pub surname: String,
+}
+
+#[derive(Debug, Clone, Hash, PartialEq, Eq, Serialize, Deserialize)]
+struct Users {
+    pub user: Vec<User>,
+}
+
+#[derive(Debug, Clone, Hash, PartialEq, Eq, Serialize, Deserialize)]
+struct TwoUsers {
+    pub user0: User,
+    pub user1: User,
+}
+
+#[test]
+fn no_unnecessary_newlines_array() {
+    assert!(!to_string(&Users {
+        user: vec![
+            User {
+                name: "John".to_string(),
+                surname: "Doe".to_string(),
+            },
+            User {
+                name: "Jane".to_string(),
+                surname: "Dough".to_string(),
+            },
+        ],
+    })
+    .unwrap()
+    .starts_with('\n'));
+}
+
+#[test]
+fn no_unnecessary_newlines_table() {
+    assert!(!to_string(&TwoUsers {
+        user0: User {
+            name: "John".to_string(),
+            surname: "Doe".to_string(),
+        },
+        user1: User {
+            name: "Jane".to_string(),
+            surname: "Dough".to_string(),
+        },
+    })
+    .unwrap()
+    .starts_with('\n'));
+}