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

.gitignore

@@ -6,6 +6,7 @@ __pycache__/
 # Node
 frontend/node_modules/
 node_modules/
+.vitest/
 
 # IDE/Misc
 .vscode/