docs: Swift functions inside types, extensions only where required (#7901)

apps/ios/CLAUDE.md

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 @README.md
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+## Swift code
+
+- Functions and computed properties are declared in the body of the type they belong to, never at file scope or in an extension of a type declared in this project.
+- A member of a SimpleXChat type is declared in that type when everything it uses is available in SimpleXChat. A member that depends on the app is a static member of the app type that uses it; when several app types use it, of the type that names its concept.
+- An extension is written only where Swift has no other form:
+  - a SwiftUI environment value, which SwiftUI requires as a property of EnvironmentValues;
+  - a conformance, required by an API, of a type declared in another module.
+- A view modifier is a ViewModifier type applied with `.modifier(_:)`.
+- A custom alignment is a static constant of a non-generic type that uses it.
+- A new member is declared in its type even when the file already extends that type. Existing extensions and file-scope functions are moved only in a task that concerns them.