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/api/client.ts

@@ -168,9 +168,9 @@ export const api = {
 
   /** Fan control */
   fans: (gpuIndex: number) => get<FanState>("/fans", gpuIndex),
-  updateFans: (curve: FanPoint[], gpuIndex: number) =>
-    post("/fans", { curve }, gpuIndex),
+  updateFans: (curve: FanPoint[], gpuIndex: number, fans?: number[] | null) =>
+    post("/fans", { curve, fans: fans ?? null }, gpuIndex),
   resetFans: (gpuIndex: number) => post("/fans/reset", undefined, gpuIndex),
-  setFanSpeed: (fanPct: number, gpuIndex: number) =>
-    post("/fans/speed", { fan_pct: fanPct }, gpuIndex),
+  setFanSpeed: (fanPct: number, gpuIndex: number, fan?: number | null) =>
+    post("/fans/speed", { fan_pct: fanPct, fan: fan ?? null }, gpuIndex),
 };