Migrate from ObservableObject to @Observable (Swift Observation)

[Demian-Yushyn]

This PR migrates ViewModels from the ObservableObject protocol and @Publishedproperties to the new Swift Observation macro @Observable.

As part of this change, the project now targets iOS 17+ as the minimum deployment version, allowing us to rely on modern SwiftUI and observation APIs without backward-compatibility workarounds.

Motivation

ObservableObject + @Published requires a significant amount of manual work and is error-prone:
properties must be explicitly marked as @Published, it’s easy to forget to publish changes, objectWillChange is implicit and not type-safe.

With iOS 17 as the minimum supported version, we can safely adopt Swift’s @Observable macro, which provides a compiler-generated, first-class observation mechanism that is:

• safer
• more concise
• easier to reason about
• officially supported and forward-looking

What changed

Before

class SomeViewModel: ObservableObject {
    @Published var value: Int = 0
}

After

@Observable
class SomeViewModel {
    var value: Int = 0
}

Benefits

• Less boilerplate
• No need for ObservableObject, @Published, or manual change notifications.
• Fewer bugs
• All mutable properties are observable by default — no risk of forgetting to publish state changes.
• Cleaner ViewModels
• Models focus on business logic instead of SwiftUI plumbing.
• Better performance characteristics
• Observation code is generated at compile time, enabling better compiler optimizations.
• Future-proof architecture
• By targeting iOS 17+, the codebase fully embraces modern SwiftUI and Swift concurrency APIs.

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