feat(oled): configurable idle-ladder thresholds (#5) + trig-fold diag counter (#6)

Issue #5 (requested by @TerryFrench): the OLED dim/off tiers are no longer
hardcoded at 2/15 min. Two new Config_body fields screen_dim_timeout /
screen_off_timeout (minutes, 0 = tier disabled, range [0,250]) are editable
on the Settings screen (ScrDim/ScrOff) and persist to flash; defaults
preserve the 2/15 ladder and upgraders read those via the config_valid clamp.
The idle timer moved from time_us_32() to 64-bit µs so the full 250-min range
is representable without the ~71-min wrap.

Issue #6: new "trig fold" Diagnostics counter — trigger-bearing 0x02 host
reports that arrived while the speaker stream was active and were folded into
the 0x36 audio frames (via state[]) rather than sent as a standalone 0x31.
Makes trig_allow == to_bt(trig share) + fold visible, confirming the apparent
trig/tx gap is audio-path folding, not dropped reports.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/config.cpp

@@ -101,6 +101,14 @@ void config_valid() {
     }
     // lb_fav_{r,g,b} need no validation — any 0..255 is a legal color, and an
     // erased flash sector (0xFF) yields 4 white favorites, a usable default.
+    if (body->screen_dim_timeout > 250) { // 0xFF erased / out of range → default
+        body->screen_dim_timeout = 2;     // mirrors the original 2-min dim tier
+        printf("[Config] screen_dim_timeout invalid, defaulting to 2 min\n");
+    }
+    if (body->screen_off_timeout > 250) {
+        body->screen_off_timeout = 15;    // mirrors the original 15-min off tier
+        printf("[Config] screen_off_timeout invalid, defaulting to 15 min\n");
+    }
     if (body->config_version != CONFIG_VERSION) {
         body->config_version = CONFIG_VERSION;
         printf("[Config] Warning: Config may breaking change\n");