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/types.ts

@@ -25,6 +25,7 @@ export interface MonitoringSample {
   temp_c: number | null;
   power_w: number | null;
   fan_pct: number | null;
+  fans: (number | null)[] | null;
   pstate: number | null;
   pstate_label: string | null;
   mem_used_bytes: number | null;
@@ -111,13 +112,22 @@ export interface FanPoint {
   fan_pct: number;
 }
 
+export interface FanInfo {
+  index: number;
+  fan_pct: number | null;
+}
+
 export interface FanState {
   fan_pct: number | null;
+  fans: FanInfo[] | null;
+  num_fans: number | null;
   fan_mode: "auto" | "curve" | null;
   min_fan_pct: number | null;
   max_fan_pct: number | null;
   curve: FanPoint[] | null;
   curve_active: boolean;
+  // Fan indices controlled by the active curve; null = all fans.
+  fan_targets: number[] | null;
 }
 
 export interface ProfileData {
@@ -127,4 +137,5 @@ export interface ProfileData {
   mem_offset_mhz: number | null;
   power_limit_w: number | null;
   fan_curve: FanPoint[] | null;
+  fan_targets: number[] | null;
 }