feat: full fan control — all fans or individual fans

The fan curve previously only controlled fan index 0; secondary fans
stayed on driver control. The curve can now target all fans (new
default) or any individual fan(s).

Backend:
- hal/fans.py: get_num_fans() via nvmlDeviceGetNumFans; get_fan_info()
  returns per-fan speeds; set_fan_speed() accepts a fan index list
  (None = all fans; all-fans mode is lenient toward driver-locked
  fans, explicit lists are strict); reset_fan() restores all fans.
- server.py: per-GPU fan_targets state; the poller applies the curve to
  all target fans and logs write failures (once per distinct error);
  activation validates targets against the hardware (stale indices fall
  back to all fans); POST /api/fans accepts fans, POST /api/fans/speed
  accepts a fan index, GET /api/fans returns num_fans/fans/fan_targets.
- Persistence format is now {"curve": ..., "fans": ...}; legacy
  bare-curve entries migrate to "all fans" at startup.
- Profiles save/apply fan_targets alongside fan_curve.
- MonitoringSample carries per-fan speeds for live gauges.

Frontend:
- Fans tab: All / Fan 1 / Fan 2 / ... selector with live per-fan %;
  the selection is applied together with the curve.
- Live monitor: per-fan gauges with sparklines for multi-fan GPUs.

frontend/src/components/Monitor/FanMonitor.tsx

@@ -47,6 +47,8 @@ export function FanMonitor({
 }: Props) {
   const fanHistory = pluck(history, "fan_pct");
   const tempHistory = pluck(history, "temp_c");
+  const fans = monitor?.fans ?? null;
+  const multiFan = fans !== null && fans.length > 1;
 
   const currentTemp = monitor?.temp_c ?? null;
   const targetFan = computeTargetFan(fanCurve, currentTemp);
@@ -68,13 +70,26 @@ export function FanMonitor({
           )}
         </div>
         <div className="flex flex-col gap-2 mt-1">
-          <GaugeCard
-            label="Fan Speed"
-            value={fmt.pct(monitor?.fan_pct)}
-            history={fanHistory}
-            color="#fb923c"
-            max={100}
-          />
+          {multiFan ? (
+            fans!.map((_, i) => (
+              <GaugeCard
+                key={i}
+                label={`Fan ${i + 1}`}
+                value={fmt.pct(fans![i])}
+                history={history.map((s) => s.fans?.[i] ?? s.fan_pct ?? 0)}
+                color="#fb923c"
+                max={100}
+              />
+            ))
+          ) : (
+            <GaugeCard
+              label="Fan Speed"
+              value={fmt.pct(monitor?.fan_pct)}
+              history={fanHistory}
+              color="#fb923c"
+              max={100}
+            />
+          )}
           <GaugeCard
             label="GPU Temp"
             value={fmt.celsius(monitor?.temp_c)}