Implement Phase 9: PDF export via share intents

- PdfExporter: creates PdfDocument, scales 300 DPI canonical coords to
  72 DPI PDF points, renders strokes without grid
- Share via Intent.ACTION_SEND + FileProvider
- PDF button in editor toolbar exports current page

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

app/src/main/kotlin/net/metacircular/engpad/ui/editor/EditorScreen.kt

@@ -14,8 +14,10 @@ import androidx.compose.ui.viewinterop.AndroidView
 import androidx.lifecycle.viewmodel.compose.viewModel
 import net.metacircular.engpad.data.db.EngPadDatabase
 import net.metacircular.engpad.data.db.toFloatArray
+import net.metacircular.engpad.data.model.Page
 import net.metacircular.engpad.data.model.PageSize
 import net.metacircular.engpad.data.repository.PageRepository
+import net.metacircular.engpad.ui.export.PdfExporter
 
 @Composable
 fun EditorScreen(
@@ -97,6 +99,16 @@ fun EditorScreen(
                 viewModel.copySelection()
                 canvasView.clearSelection()
             },
+            onExport = {
+                val page = Page(id = pageId, notebookId = 0, pageNumber = 1, createdAt = 0)
+                val file = PdfExporter.exportPages(
+                    context = context,
+                    notebookTitle = "eng-pad-export",
+                    pageSize = pageSize,
+                    pages = listOf(page to strokes),
+                )
+                PdfExporter.shareFile(context, file)
+            },
             modifier = Modifier.fillMaxWidth(),
         )
         AndroidView(

app/src/main/kotlin/net/metacircular/engpad/ui/editor/Toolbar.kt

@@ -23,6 +23,7 @@ fun EditorToolbar(
     hasSelection: Boolean,
     onDeleteSelection: () -> Unit,
     onCopySelection: () -> Unit,
+    onExport: () -> Unit,
     modifier: Modifier = Modifier,
 ) {
     Row(
@@ -58,6 +59,7 @@ fun EditorToolbar(
             TextButton(onClick = onCopySelection) { Text("Copy") }
         }
         Spacer(modifier = Modifier.weight(1f))
+        TextButton(onClick = onExport) { Text("PDF") }
         TextButton(onClick = onUndo, enabled = canUndo) {
             Text("Undo")
         }

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

@@ -0,0 +1,102 @@
+package net.metacircular.engpad.ui.export
+
+import android.content.Context
+import android.content.Intent
+import android.graphics.Canvas
+import android.graphics.Color
+import android.graphics.Paint
+import android.graphics.Path
+import android.graphics.pdf.PdfDocument
+import androidx.core.content.FileProvider
+import net.metacircular.engpad.data.db.toFloatArray
+import net.metacircular.engpad.data.model.Page
+import net.metacircular.engpad.data.model.PageSize
+import net.metacircular.engpad.data.model.Stroke
+import java.io.File
+
+/**
+ * Exports pages to PDF. Coordinates are in 300 DPI canonical space and
+ * scaled to 72 DPI (PDF points) during export via a scale factor of 0.24.
+ */
+object PdfExporter {
+
+    private const val CANONICAL_TO_PDF = 72f / 300f // 0.24
+
+    fun exportPages(
+        context: Context,
+        notebookTitle: String,
+        pageSize: PageSize,
+        pages: List<Pair<Page, List<Stroke>>>,
+    ): File {
+        val pdfDoc = PdfDocument()
+
+        val pdfWidth = (pageSize.widthPt * CANONICAL_TO_PDF).toInt()
+        val pdfHeight = (pageSize.heightPt * CANONICAL_TO_PDF).toInt()
+
+        for ((index, pair) in pages.withIndex()) {
+            val (_, strokes) = pair
+            val pageInfo = PdfDocument.PageInfo.Builder(pdfWidth, pdfHeight, index + 1).create()
+            val pdfPage = pdfDoc.startPage(pageInfo)
+            val canvas = pdfPage.canvas
+
+            // Scale from canonical (300 DPI) to PDF (72 DPI)
+            canvas.scale(CANONICAL_TO_PDF, CANONICAL_TO_PDF)
+
+            // Draw strokes (no grid)
+            for (stroke in strokes) {
+                val points = stroke.pointData.toFloatArray()
+                val path = buildPath(points)
+                val paint = buildPaint(stroke.penSize, stroke.color)
+                canvas.drawPath(path, paint)
+            }
+
+            pdfDoc.finishPage(pdfPage)
+        }
+
+        val exportDir = File(context.cacheDir, "exports")
+        exportDir.mkdirs()
+        val sanitizedTitle = notebookTitle.replace(Regex("[^a-zA-Z0-9._-]"), "_")
+        val file = File(exportDir, "$sanitizedTitle.pdf")
+        file.outputStream().use { pdfDoc.writeTo(it) }
+        pdfDoc.close()
+
+        return file
+    }
+
+    fun shareFile(context: Context, file: File) {
+        val uri = FileProvider.getUriForFile(
+            context,
+            "${context.packageName}.fileprovider",
+            file,
+        )
+        val intent = Intent(Intent.ACTION_SEND).apply {
+            type = "application/pdf"
+            putExtra(Intent.EXTRA_STREAM, uri)
+            addFlags(Intent.FLAG_GRANT_READ_URI_PERMISSION)
+        }
+        context.startActivity(Intent.createChooser(intent, "Export PDF"))
+    }
+
+    private fun buildPath(points: FloatArray): Path {
+        val path = Path()
+        if (points.size < 2) return path
+        path.moveTo(points[0], points[1])
+        var i = 2
+        while (i < points.size - 1) {
+            path.lineTo(points[i], points[i + 1])
+            i += 2
+        }
+        return path
+    }
+
+    private fun buildPaint(penSize: Float, color: Int): Paint {
+        return Paint().apply {
+            this.color = color
+            strokeWidth = penSize
+            style = Paint.Style.STROKE
+            strokeCap = Paint.Cap.ROUND
+            strokeJoin = Paint.Join.ROUND
+            isAntiAlias = true
+        }
+    }
+}