feat: wireview pin-imbalance warning, per-pin rating bars, pin_imbalance_a

- Per-pin bar now scales against the 9.2 A per-pin rating (10 A full
  scale, red at/over rating) instead of the 55 A total connector OCP
- Debounced app-wide warning banner (visible on all tabs) when the
  current between two pins diverges by > 2 A for ~7 s - the signature
  of a bad pin contact that can overheat and melt the 12VHPWR
  connector; 5 s grace period after clearing
- New pin_imbalance_a sample field (max - min pin current) in both the
  serial and hwmon sample builders, plus frontend type
- Docs: README + WireView.md cover the warning and threshold; fresh
  screenshot with the new per-pin labels
- Tests: vitest setup + 9 unit tests for the debounce state machine;
  3 new backend checks for pin_imbalance_a

docs/WireView.md

@@ -74,6 +74,12 @@ ls -l /dev/wireview-pro2       # -> /dev/ttyACM0
 
 With the device plugged in, start (or restart) the NVCurve server. The **WireView** tab appears in the web UI and `GET /api/wireview` returns the device info and live samples. The tab disappears when the device is unplugged and reappears on re-plug — no restart needed.
 
+## Pin current imbalance warning
+
+The 12 VHPWR connector is the most fragile part of a PC's power delivery: if one power pin has a bad contact (worn cable, faulty adapter, loose plug), the remaining pins are forced to carry the missing current. Under heavy load this can overheat and melt the connector — in rare cases even start a fire.
+
+NVCurve watches for this: if the current between any two power pins diverges by more than **2 A**, a big warning appears in the app header (visible on every tab, not just WireView) naming the deviating pins (e.g. `Pin 5 (5.6 A) vs Pin 6 (3.5 A)`). It is debounced — the imbalance must persist for several seconds (~7 s) before the warning shows, so single-sample glitches and short load ramps (e.g. AI workloads) do not flash it. When you see it, reduce the load and replace the cable or adapter — do not keep using the connector.
+
 ## Alternative: wireview-hwmon kernel module
 
 If the official `wireview-hwmon` kernel module and its `wireviewd` daemon are installed, the daemon owns the serial port and NVCurve automatically reads the sysfs node instead. No udev rule is needed in that case.