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.

docs/Overview.md

@@ -17,7 +17,7 @@ NVCurve provides two ways to interact with your GPU:
 
 - **Per-Point Curve Editing** — Adjust the frequency offset for any individual voltage point on the V/F curve.
 - **Extended Memory Offset** — Memory clock offset up to +3000 MHz (Blackwell GPUs). Standard NVCurve caps at +1000 MHz.
-- **Fan Curve Control** — Custom temperature-to-fan-speed curves via NVML, adjustable through the web UI and savable in profiles.
+- **Fan Curve Control** — Custom temperature-to-fan-speed curves via NVML (all fans or individual fans), adjustable through the web UI and savable in profiles.
 - **Curve Flattening** — Select multiple points and flatten them to a common frequency using anchor-point targeting.
 - **Live Monitoring** — Track GPU voltage, clock speed, temperature, and power draw in real time via NvAPI and NVML.
 - **Profile Management** — Save, load, and switch between named profiles. Set a default profile that auto-applies on startup.
@@ -29,7 +29,7 @@ NVCurve provides two ways to interact with your GPU:
 NVCurve consists of two components:
 
 | Component | Description |
-|---|---|
+| --- | --- |
 | **Python Backend** | Talks directly to `libnvidia-api.so` (via ctypes) and `libnvidia-ml.so` to read and write GPU hardware state. Exposes functionality through a FastAPI REST + WebSocket server. |
 | **React Frontend** | Runs in the browser and communicates with the backend over HTTP and WebSockets. Handles curve visualization, point editing, live monitoring, and profile management. |