fix: crop rotated images, radius in px, modals block timeline

Replays the 13 open bug reports on alyssaxuu/motionity against this fork.
Nine were already fixed by the earlier audit and one (#8, video from the
search tab) needs a Pixabay API key to reproduce; these are the three that
were still broken.

#23 - border radius behaved like a percentage. fabric applies rx/ry before
the object's scale and shapes are resized by scaling, so a radius typed as
20 drew at 60px on a rect scaled 3x while the panel still read 20. A new
cornerRadius property keeps the pixel value the user asked for, rx/ry are
derived from it and re-derived whenever the scale changes, and old projects
fall back to rx * scaleX.

#27 - cropping a rotated image cropped the wrong region. crop() compared
canvas-space edges and covered only three of the four quadrants, so it did
nothing at all when the crop window was centred on the image. It now works
in the image's own frame, clamps the region to the bitmap and re-centres the
object on what it kept; the crop window carries the image's angle and the
expand-to-full-bitmap shift in cropImage is rotated the same way.

#1 - modals are painted over the timeline but never took its pointer events,
so the resize handle, seekbar, keyframes and layer bars all reacted to a drag
behind the dialog. All four now bail while a modal is open.

Verified in Chromium: 20px draws a 20px corner at scale 1 and scale 3 and
survives save, JSON round-trip and reload; at 0, 30 and 45 degrees the pixels
under the crop window are unchanged by the crop and everything outside it is
dropped; a real drag on the timeline handle with the export modal open leaves
the timeline height untouched. WEBM export and a non-rotated crop still work.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/js/database.js

@@ -50,6 +50,7 @@ function checkDB() {
           'strokeUniform',
           'rx',
           'ry',
+          'cornerRadius',
           'selectable',
           'hasControls',
           'subTargetCheck',
@@ -226,6 +227,7 @@ async function autoSave() {
       'strokeUniform',
       'rx',
       'ry',
+      'cornerRadius',
       'selectable',
       'hasControls',
       'subTargetCheck',