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/postcard/README.md

@@ -0,0 +1,146 @@
+# Postcard
+
+[![Documentation](https://docs.rs/postcard/badge.svg)](https://docs.rs/postcard)
+
+Postcard is a `#![no_std]` focused serializer and deserializer for Serde.
+
+Postcard aims to be convenient for developers in constrained environments, while
+allowing for flexibility to customize behavior as needed.
+
+## Design Goals
+
+1. Design primarily for `#![no_std]` usage, in embedded or other constrained contexts
+2. Support a maximal set of `serde` features, so `postcard` can be used as a drop in replacement
+3. Avoid special differences in code between communication code written for a microcontroller or a desktop/server PC
+4. Be resource efficient - memory usage, code size, developer time, and CPU time; in that order
+5. Allow library users to customize the serialization and deserialization  behavior to fit their bespoke needs
+
+## Format Stability
+
+As of v1.0.0, `postcard` has a documented and stable wire format. More information about this
+wire format can be found in the `spec/` folder of the Postcard repository, or viewed online
+at <https://postcard.jamesmunns.com>.
+
+Work towards the Postcard Specification and portions of the Postcard 1.0 Release
+were sponsored by Mozilla Corporation.
+
+## Variable Length Data
+
+All signed and unsigned integers larger than eight bits are encoded using a [Varint].
+This includes the length of array slices, as well as the discriminant of `enums`.
+
+For more information, see the [Varint] chapter of the wire specification.
+
+[Varint]: https://postcard.jamesmunns.com/wire-format.html#varint-encoded-integers
+
+## Example - Serialization/Deserialization
+
+Postcard can serialize and deserialize messages similar to other `serde` formats.
+
+Using the default `heapless` feature to serialize to a `heapless::Vec<u8>`:
+
+```rust
+use core::ops::Deref;
+use serde::{Serialize, Deserialize};
+use postcard::{from_bytes, to_vec};
+use heapless::Vec;
+
+#[derive(Serialize, Deserialize, Debug, Eq, PartialEq)]
+struct RefStruct<'a> {
+    bytes: &'a [u8],
+    str_s: &'a str,
+}
+let message = "hElLo";
+let bytes = [0x01, 0x10, 0x02, 0x20];
+let output: Vec<u8, 11> = to_vec(&RefStruct {
+    bytes: &bytes,
+    str_s: message,
+}).unwrap();
+
+assert_eq!(
+    &[0x04, 0x01, 0x10, 0x02, 0x20, 0x05, b'h', b'E', b'l', b'L', b'o',],
+    output.deref()
+);
+
+let out: RefStruct = from_bytes(output.deref()).unwrap();
+assert_eq!(
+    out,
+    RefStruct {
+        bytes: &bytes,
+        str_s: message,
+    }
+);
+```
+
+Or the optional `alloc` feature to serialize to an `alloc::vec::Vec<u8>`:
+
+```rust
+use core::ops::Deref;
+use serde::{Serialize, Deserialize};
+use postcard::{from_bytes, to_allocvec};
+extern crate alloc;
+use alloc::vec::Vec;
+
+#[derive(Serialize, Deserialize, Debug, Eq, PartialEq)]
+struct RefStruct<'a> {
+    bytes: &'a [u8],
+    str_s: &'a str,
+}
+let message = "hElLo";
+let bytes = [0x01, 0x10, 0x02, 0x20];
+let output: Vec<u8> = to_allocvec(&RefStruct {
+    bytes: &bytes,
+    str_s: message,
+}).unwrap();
+
+assert_eq!(
+    &[0x04, 0x01, 0x10, 0x02, 0x20, 0x05, b'h', b'E', b'l', b'L', b'o',],
+    output.deref()
+);
+
+let out: RefStruct = from_bytes(output.deref()).unwrap();
+assert_eq!(
+    out,
+    RefStruct {
+        bytes: &bytes,
+        str_s: message,
+    }
+);
+```
+
+## Flavors
+
+`postcard` supports a system called `Flavors`, which are used to modify the way
+postcard serializes or processes serialized data. These flavors act as "plugins" or "middlewares"
+during the serialization or deserialization process, and can be combined to obtain complex protocol formats.
+
+See the documentation of the `ser_flavors` or `de_flavors` modules for more information on usage.
+
+## Setup - `Cargo.toml`
+
+Don't forget to add [the `no-std` subset](https://serde.rs/no-std.html) of `serde` along with `postcard` to the `[dependencies]` section of your `Cargo.toml`!
+
+```toml
+[dependencies]
+postcard = "1.0.0"
+
+# By default, `serde` has the `std` feature enabled, which makes it unsuitable for embedded targets
+# disabling default-features fixes this
+serde = { version = "1.0.*", default-features = false }
+```
+
+## License
+
+Licensed under either of
+
+- Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or
+  <http://www.apache.org/licenses/LICENSE-2.0>)
+- MIT license ([LICENSE-MIT](LICENSE-MIT) or <http://opensource.org/licenses/MIT>)
+
+at your option.
+
+### Contribution
+
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in the work by you, as defined in the Apache-2.0 license, shall be
+dual licensed as above, without any additional terms or conditions.