feat(display): OLED dims a few seconds after last activity

Splash holds bright for 10 s on (re-)entry, then drops to dim while
LearnMore/Info keep alternating. The dashboard stays bright for 3 s
after the last packet or mode change, then dims in place without
disturbing the Listening/Transmitting state or the 1 s sparkline
timer. Cross-leaf TX↔RX hops stay bright throughout — `set_dim` is
fired by the 3 s timer handler as an action rather than as a sibling
`exit:` action, so mode-change leaf-crossings no longer flash
dim→bright on every swap.

Two driver families (`ssd1306::Ssd1306Async` and the in-tree
`Sh1106`) wrapped behind a single `DisplayBrightness` trait in
`src/driver/mod.rs`; the board trait's `DisplayDriver` associated
type now requires it. Dim/bright contrast values centralized as
`CONTRAST_BRIGHT` / `CONTRAST_DIM` constants — tune in one place.

`On.on(CmdReset)` now targets `LearnMore` directly rather than the
ancestor `Splash`. Under hsmc 0.2 up-transition semantics, targeting
an already-active ancestor unwinds only below it without re-descent;
the USB-disconnect reset path was previously relying on the old
re-enter-and-descend behavior and would have parked at `Splash`
with a dead alternation timer.

Bumps `hsmc` 0.1.0 → 0.2 for the up-transition semantic fix.

Cargo.toml

@@ -31,7 +31,7 @@ embassy-usb-driver = "0.2"
 ucobs = { version = "0.3", features = ["legacy-msrv"] }
 heapless = "0.9"
 donglora-protocol = "1.1.0"
-hsmc = { version = "0.1.0", default-features = false, features = ["embassy"] }
+hsmc = { version = "0.2", default-features = false, features = ["embassy"] }
 lora-phy = "3.0"
 portable-atomic = { version = "1", optional = true, default-features = false, features = ["critical-section"] }
 static_cell = "2.1"