[Refactor] Move all icons to common `StudyBuddyIcons` object

[EdricChan03]

Part of the Compose migration #368, consider migrating all vector drawable-based icons to their Compose ImageVector equivalent.

We can use the Valkyire IDE plugin for this, which can generate the following structure:

object StudyBuddyIcons {
  object Filled
  object Outlined
}

val StudyBuddyIcons.Filled.Help: ImageVector by lazy { /* ... */ }

Tasks

• Add :ui:icons (or :core:designsystem:icons)