docs: Add QA testing checklist

Author: jacobgkau

TESTING.md

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+# Testing
+
+This document provides a regression testing checklist for COSMIC Files. The checklist provides a starting point for Quality Assurance reviews.
+
+## Checklist
+
+### Basic navigation
+
+- [ ] Middle-click opens directory in a new tab (not focused).
+- [ ] Open two scrollable tabs. Scroll one tab, then switch to the other tab; it should not have scrolled.
+- [ ] Hover over the top item in the folder, then scroll down until it's out of view (while still hovered).
+      On scrolling back up (with the mouse in a different position), the item should not have the hover highlight.
+- [ ] Right-click an item in the sidebar. No visual change should occur with the rest of the items.
+- [ ] Remove an item from the sidebar, then re-pin it.
+
+### File operations
+
+- [ ] Right-click -> Create a new folder, then enter it.
+- [ ] Right-click in the empty folder -> Create a new file.
+- [ ] Normal right-click shows `Move to trash` option.
+- [ ] Shift right-click, and right-click followed by Shift, both show `Permanently delete` option.
+
+### Advanced navigation & view settings
+
+- [ ] Image and video thumbnails display in local folders.
+- [ ] Gallery preview shows with Spacebar.
+- [ ] Details pane shows with Ctrl+Spacebar.
+- [ ] Zoom in/out and reset to default zoom work.
+- [ ] Ctrl+1 and Ctrl+2 switch between list and icon view.
+- [ ] Ctrl+H shows/hides hidden files.
+- [ ] Directories can be sorted at top or inline.
+- [ ] Settings -> Theme works.
+- [ ] Settings -> Type to Search affects behavior as designed.
+- [ ] Single-click to open setting takes effect.
+- [ ] Sorting options work.
+- [ ] Cutting, copying, and pasting files works.
+- [ ] F5 reloads current directory.
+- [ ] Left sidebar can be collapsed and expanded.
+
+### External filesystems
+
+- [ ] Add a network drive (e.g. SFTP) and navigate into it.
+- [ ] Plug in a USB drive; able to mount, browse, and eject.