Enforce minimum zoom so page always fills viewport

Dynamic min zoom computed from page and view dimensions — page can
never be smaller than the visible area. Combined with pan clamping
from previous commit, the page always covers the full canvas with
no non-drawable area visible.

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

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

@@ -93,7 +93,7 @@ class PadCanvasView(context: Context) : View(context) {
         object : ScaleGestureDetector.SimpleOnScaleGestureListener() {
             override fun onScale(detector: ScaleGestureDetector): Boolean {
                 val newZoom = (zoom * detector.scaleFactor)
-                    .coerceIn(CanvasState.MIN_ZOOM, CanvasState.MAX_ZOOM)
+                    .coerceIn(minZoom(), CanvasState.MAX_ZOOM)
                 if (newZoom != zoom) {
                     val focusX = detector.focusX
                     val focusY = detector.focusY
@@ -639,6 +639,23 @@ class PadCanvasView(context: Context) : View(context) {
 
     // --- Coordinate transforms ---
 
+    /**
+     * Minimum zoom so the page always fills the viewport.
+     * At fitScale (zoom=1), the page fills the screen width.
+     * We need both dimensions covered, so min zoom = max(1, viewH/(pageH*fitScale)).
+     */
+    private fun minZoom(): Float {
+        val pageW = canvasState.pageSize.widthPt.toFloat()
+        val pageH = canvasState.pageSize.heightPt.toFloat()
+        val viewW = width.toFloat().coerceAtLeast(1f)
+        val viewH = height.toFloat().coerceAtLeast(1f)
+        val fitScale = viewW / pageW
+        // Page must fill both dimensions
+        val minForWidth = 1f  // zoom=1 means page width = view width
+        val minForHeight = viewH / (pageH * fitScale)
+        return maxOf(minForWidth, minForHeight)
+    }
+
     private fun rebuildViewMatrix() {
         viewMatrix.reset()
         val pageW = canvasState.pageSize.widthPt.toFloat()