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.

README.md

@@ -13,7 +13,7 @@ NVCurve brings MSI Afterburner-style per-point voltage-frequency curve control t
 
 > [!IMPORTANT]
 > **Blackwell GPU memory** — This is a specialized fork with extended memory offset support (up to +3000 MHz) for Blackwell GPUs (RTX 50-series). \
-> **Fan Controls** — There is an additional "Fans" tab to setup a customized fan curve. \
+> **Fan Controls** — There is an additional "Fans" tab to setup a customized fan curve, controlling all fans or individual fans. \
 > **Dashboard** — The default tab is an Dashboard with additional information (PCIe link speed, VBIOS information, Max Core Clock, Throttle Reason and much much more.) \
 > **Authentification** — For production deplyoment, I added authentification with bcrypt hashing to allow only one or multiple people to have access. \
 > Installing the pre-built PyPI package will NOT include these features. You must build from source.