feat: add tasks and notes management for lessons

- Introduced a new feature for managing tasks and notes associated with lessons.
- Added a `NotesStore` class to handle local storage of notes in JSON format.
- Implemented UI components for displaying, adding, editing, and deleting notes.
- Enhanced the `HomeScreen`, `ScheduleScreen`, and `SubjectsScreen` to integrate notes functionality.
- Updated the API to handle versioning and added error handling for outdated endpoints.
- Improved the overall user experience with visual indicators for tasks and notes in the schedule.

android/app/src/main/kotlin/ritm/schedule/data/Notes.kt

@@ -0,0 +1,59 @@
+package ritm.schedule.data
+
+import android.content.Context
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.sync.Mutex
+import kotlinx.coroutines.sync.withLock
+import kotlinx.coroutines.withContext
+import kotlinx.serialization.Serializable
+import java.io.File
+import java.util.UUID
+
+/**
+ * A homework task or a plain note the student pinned to one lesson: the subject,
+ * the lesson's date and start time. Tasks can be ticked off; notes just remind.
+ */
+@Serializable
+data class Note(
+    val id: String = UUID.randomUUID().toString(),
+    /** "docId|group": notes of one group never show up in another. */
+    val group: String,
+    val subject: String,
+    val date: String,
+    val start: String,
+    val text: String,
+    val task: Boolean = true,
+    val done: Boolean = false,
+    val created: Long = System.currentTimeMillis(),
+)
+
+fun groupKey(g: SavedGroup) = "${g.docId}|${g.group}"
+
+/** Notes live only on the device, in one small JSON file. */
+class NotesStore(context: Context) {
+    private val file = File(context.filesDir, "notes.json")
+    private val lock = Mutex()
+    private val _notes = MutableStateFlow(read())
+    val notes: StateFlow<List<Note>> = _notes
+
+    private fun read(): List<Note> =
+        runCatching { json.decodeFromString<List<Note>>(file.readText()) }.getOrDefault(emptyList())
+
+    private suspend fun change(block: (List<Note>) -> List<Note>) = withContext(Dispatchers.IO) {
+        lock.withLock {
+            val next = block(_notes.value)
+            file.writeText(json.encodeToString(next))
+            _notes.value = next
+        }
+    }
+
+    suspend fun save(note: Note) = change { list ->
+        if (list.any { it.id == note.id }) list.map { if (it.id == note.id) note else it } else list + note
+    }
+
+    suspend fun delete(id: String) = change { list -> list.filterNot { it.id == id } }
+
+    suspend fun toggle(id: String) = change { list -> list.map { if (it.id == id) it.copy(done = !it.done) else it } }
+}