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/litrs/rustfmt.toml

@@ -0,0 +1,11 @@
+# Rustfmt is not fully used with this crate, as its formatting decreases
+# readability in too many places. Therefore formatting is disabled. This config
+# file exists for me to occasionally run `cargo fmt` and cherry pick changes.
+disable_all_formatting = true
+
+blank_lines_upper_bound = 3
+group_imports = "StdExternalCrate"
+use_field_init_shorthand = true
+overflow_delimited_expr = true
+single_line_if_else_max_width = 60
+struct_lit_width = 70