Add notebook zip backup export

- BackupExporter: exports notebook as .engpad.zip containing
  notebook.json (metadata) and pages/NNN.json (strokes as JSON
  float arrays). Uses Dispatchers.IO for file I/O.
- Added "Export backup" to notebook overflow menu in library
- PdfExporter.sanitize changed from private to internal for reuse
- Shares via FileProvider + ACTION_SEND with application/zip MIME type

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/src/main/kotlin/net/metacircular/engpad/ui/export/BackupExporter.kt

@@ -0,0 +1,71 @@
+package net.metacircular.engpad.ui.export
+
+import android.content.Context
+import net.metacircular.engpad.data.db.toFloatArray
+import net.metacircular.engpad.data.repository.NotebookRepository
+import net.metacircular.engpad.data.repository.PageRepository
+import org.json.JSONArray
+import org.json.JSONObject
+import java.io.File
+import java.util.zip.ZipEntry
+import java.util.zip.ZipOutputStream
+
+object BackupExporter {
+
+    suspend fun exportNotebook(
+        context: Context,
+        notebookId: Long,
+        notebookRepository: NotebookRepository,
+        pageRepository: PageRepository,
+    ): File {
+        val notebook = notebookRepository.getById(notebookId)
+            ?: throw IllegalArgumentException("Notebook $notebookId not found")
+
+        val pages = pageRepository.getPagesList(notebookId)
+            .sortedBy { it.pageNumber }
+
+        val exportDir = File(context.cacheDir, "exports")
+        exportDir.mkdirs()
+        val sanitizedTitle = PdfExporter.sanitize(notebook.title)
+        val file = File(exportDir, "$sanitizedTitle.engpad.zip")
+
+        ZipOutputStream(file.outputStream()).use { zip ->
+            val notebookJson = JSONObject().apply {
+                put("title", notebook.title)
+                put("page_size", notebook.pageSize)
+                put("page_count", pages.size)
+            }
+            zip.putNextEntry(ZipEntry("notebook.json"))
+            zip.write(notebookJson.toString(2).toByteArray())
+            zip.closeEntry()
+
+            for (page in pages) {
+                val strokes = pageRepository.getStrokes(page.id)
+                val strokesJson = JSONArray()
+                for (stroke in strokes) {
+                    val points = stroke.pointData.toFloatArray()
+                    val pointsArray = JSONArray()
+                    for (p in points) {
+                        pointsArray.put(p.toDouble())
+                    }
+                    strokesJson.put(JSONObject().apply {
+                        put("pen_size", stroke.penSize.toDouble())
+                        put("color", stroke.color)
+                        put("style", stroke.style)
+                        put("points", pointsArray)
+                    })
+                }
+                val pageJson = JSONObject().apply {
+                    put("page_number", page.pageNumber)
+                    put("strokes", strokesJson)
+                }
+                val entryName = "pages/%03d.json".format(page.pageNumber)
+                zip.putNextEntry(ZipEntry(entryName))
+                zip.write(pageJson.toString(2).toByteArray())
+                zip.closeEntry()
+            }
+        }
+
+        return file
+    }
+}

app/src/main/kotlin/net/metacircular/engpad/ui/export/PdfExporter.kt

@@ -124,6 +124,6 @@ object PdfExporter {
         }
     }
 
-    private fun sanitize(name: String): String =
+    internal fun sanitize(name: String): String =
         name.replace(Regex("[^a-zA-Z0-9._-]"), "_")
 }

app/src/main/kotlin/net/metacircular/engpad/ui/notebooks/NotebookListScreen.kt

@@ -31,16 +31,24 @@ import androidx.compose.runtime.collectAsState
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
+import androidx.compose.runtime.rememberCoroutineScope
 import androidx.compose.runtime.setValue
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
+import androidx.compose.ui.platform.LocalContext
 import androidx.compose.ui.text.input.KeyboardCapitalization
 import androidx.compose.ui.unit.dp
 import androidx.lifecycle.viewmodel.compose.viewModel
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.launch
+import kotlinx.coroutines.withContext
 import net.metacircular.engpad.data.db.EngPadDatabase
 import net.metacircular.engpad.data.model.Notebook
 import net.metacircular.engpad.data.model.PageSize
 import net.metacircular.engpad.data.repository.NotebookRepository
+import net.metacircular.engpad.data.repository.PageRepository
+import net.metacircular.engpad.ui.export.BackupExporter
+import net.metacircular.engpad.ui.export.PdfExporter
 import java.text.SimpleDateFormat
 import java.util.Date
 import java.util.Locale
@@ -62,6 +70,9 @@ fun NotebookListScreen(
     val repository = remember {
         NotebookRepository(database.notebookDao(), database.pageDao())
     }
+    val pageRepository = remember {
+        PageRepository(database.pageDao(), database.strokeDao())
+    }
     val viewModel: NotebookListViewModel = viewModel(
         factory = NotebookListViewModel.Factory(repository),
     )
@@ -69,6 +80,8 @@ fun NotebookListScreen(
     var showCreateDialog by remember { mutableStateOf(false) }
     var notebookToDelete by remember { mutableStateOf<Notebook?>(null) }
     var notebookToRename by remember { mutableStateOf<Notebook?>(null) }
+    val context = LocalContext.current
+    val scope = rememberCoroutineScope()
 
     var filterText by remember { mutableStateOf("") }
     var sortField by remember { mutableStateOf(SortField.LAST_EDITED) }
@@ -171,6 +184,16 @@ fun NotebookListScreen(
                             onClick = { onNotebookClick(notebook.id) },
                             onRename = { notebookToRename = notebook },
                             onDelete = { notebookToDelete = notebook },
+                            onExportBackup = {
+                                scope.launch {
+                                    val file = withContext(Dispatchers.IO) {
+                                        BackupExporter.exportNotebook(
+                                            context, notebook.id, repository, pageRepository,
+                                        )
+                                    }
+                                    PdfExporter.shareFile(context, file, "application/zip")
+                                }
+                            },
                         )
                     }
                 }
@@ -217,6 +240,7 @@ private fun NotebookItem(
     onClick: () -> Unit,
     onRename: () -> Unit,
     onDelete: () -> Unit,
+    onExportBackup: () -> Unit,
 ) {
     val dateFormat = remember { SimpleDateFormat("yyyy-MM-dd HH:mm", Locale.US) }
     var showMenu by remember { mutableStateOf(false) }
@@ -262,6 +286,10 @@ private fun NotebookItem(
                         text = { Text("Rename") },
                         onClick = { showMenu = false; onRename() },
                     )
+                    DropdownMenuItem(
+                        text = { Text("Export backup") },
+                        onClick = { showMenu = false; onExportBackup() },
+                    )
                     DropdownMenuItem(
                         text = { Text("Delete") },
                         onClick = { showMenu = false; onDelete() },