fix(v2/hcore): prevent panic in StartService when HiddifyOptions is nil

[nisemenov]

Problem

StartService may panic when static.HiddifyOptions is nil.

startmobile panic: runtime error:
invalid memory address or nil pointer dereference goroutine 15 [running]: runtime/debug.Stack()
runtime/debug/stack.go:26 +0x64
github.com/hiddify/hiddify-core/v2/
config. DeferPanicToError ({0x105bb e11f, Oxb}, 0x13006b818)
github.com/hiddify/hiddify-core/v2/
config/debug.go:30 +0x44
panic ({0x106181620?, 0x107609ba0?})
runtime/panic.go:783 +0x120
github.com/hiddify/hiddify-core/v2/
config. BuildConfig ({0x10656e3c8?,
0x13022c090?}, Оx0, 0x1?)
github.com/hiddify/hiddify-core/v2/
config/builder.go:70 +0x50
github.com/hiddify/hiddify-core/v2/
hcore.BuildConfig({0x10656e3c8,
0x13022c090}, 0x1302867e0)
github.com/hiddify/hiddify-core/v2/
hcore/buildconfighelper.go:40

This can occur in some environments where StartService is invoked
before internal state is fully initialized (e.g. app lifecycle edge cases).

Solution

• Add a guard in StartService to check for nil HiddifyOptions
• Return a proper error instead of allowing a panic

Notes

This makes the startup logic more robust against invalid state without changing deeper configuration logic.