feat(agenda): Implement Part 1 - DTOs and Dependencies

[DemisChan]

This MR introduces the foundational data structures and required dependencies for the new Agenda
feature. It constitutes Part 1 of the feature implementation, keeping the change set small and
focused for easier review.

Key Changes:

1. Data Transfer Objects (DTOs):

   • Creates all necessary DTOs for Events, Tasks, Reminders, and related entities (Photos,
     Attendees).
   • This establishes the data contract with the remote API as defined in the requirements.

2. Dependencies:

   • Adds io.coil-kt:coil-compose to support future image loading.
   • Adds com.google.accompanist:accompanist-permissions to prepare for handling photo
     selection and other runtime permissions.

[Kai-Cilliers]

I personally got away from using mocks as much as possible. They're good in two scenarios:

1. You're testing legacy code which isn't written with testability in mind.
2. You're testing code you don't own (e.g. you need to make a library's function return something specific and it's not extendable)

In all other cases, I found fakes/stubs much more reliable and less error prone. I can't count how often my tests broke at some point, not because there was a bug, but because the mock itself broke (for example when the mocked functionality changed/was extended, there were unmocked function calls, etc.).
Having a fake instead will make you implement new functionality before you can even run your tests and also doesn't rely on that much 3rd party library magic as it's just a plain Kotlin class.

[Kai-Cilliers]

Alternatively, you could think about creating a sealed hierarchy just for the differences of the agenda items. This is what I mean:

sealed interface AgendaItemDetails {
    data class Event(
        val toTime: LocalDateTime,
        val attendees: List<Attendee>,
        val photos: List<Photo>
    ): AgendaItemDetails
    
    data class Task(
        val isDone: Boolean
    ): AgendaItemDetails
    
    data object Reminder: AgendaItemDetails
}

That way, you just need type-checks when updating properties that are individual to one of the agenda item types, but you don't need this for updating shared properties anymore like the title:

data class AgendaDetailState(
    val title: String = "",
    val description: String = "",
    val time: LocalDateTime = LocalDateTime.now(),
    val details: AgendaItemDetails? = null
)

Updating then works like this:

// Updating a shared property
state = state.copy(title = "new title")

// Updating an individual property
state = state.copy(
    details = detailsAsEvent()?.copy(attendees = newAttendees)
)