Polish media viewer, pull-to-sync, search nav, and add seek bar presets

- Remove peek details sheet from media viewer; scope Activity tab to
  the current file via client-side filtering instead of unreliable
  server-side query params
- Add file rename action (WebDAV MOVE); fix search result taps to
  switch to Files tab and rebuild breadcrumbs from an absolute path
- Add swipe-to-delete confirmation dialog; fix a crash from disposing
  the rename dialog's TextEditingController too early
- Rework pull-to-sync gesture so sync and lock-open thresholds fire
  independently within one gesture, raise thresholds, and add a
  classic Material refresh spinner
- Add "tap active tab to scroll to top" behavior with a settings toggle
- Align selection toolbar padding with the sort/filter controls row
- Replace app icon assets (adaptive icon + splash screen) with
  updated IconKitchen export
- Replace the seek bar style dropdown with a Default/Wavy/Slim/Squiggly
  preset picker, each rendered with a live, perpetually-animated
  preview via a new shared SeekBarPainter/SeekBarPreview

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

pubspec.yaml

@@ -84,6 +84,9 @@ flutter:
   # the material Icons class.
   uses-material-design: true
 
+  assets:
+    - assets/icon/app_icon.png
+
   # To add assets to your application, add an assets section, like this:
   # assets:
   #   - images/a_dot_burr.jpeg