feat: auto-detect OS from description; expand OS_LIST to 51 distros

- Remove os field from Device API and form — OS is now detected
  automatically from name/description via detectOs() in brandIcons.js
- Expand OS_LIST from 20 to 51 entries covering all major distros
  (Debian/Ubuntu flavours, Red Hat, SUSE, Arch, BSD, security distros,
  Windows/macOS/iOS/Android, generic Linux/BSD catch-alls)
- Display detected OS icon in IpAddressing.vue description column
- Fix virt_type validator to normalise empty string to null

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/routers/devices.py

@@ -91,6 +91,8 @@ class DeviceCreate(BaseModel):
     @field_validator("virt_type")
     @classmethod
     def _virt_type(cls, v: Optional[str]) -> Optional[str]:
+        if not v:
+            return None
         if v not in _VALID_VIRT_TYPES:
             raise ValueError(f"Invalid virt_type: {v!r}. Must be one of: baremetal, lxc, qemu")
         return v
@@ -104,7 +106,6 @@ class DeviceCreate(BaseModel):
                 raise ValueError("url must be a valid http or https URL")
         return v
 
-
 class DeviceOut(BaseModel):
     id: int
     name: str