fix: misc. minor big fixes

[dlyongemallo]

• Clear loading flags on error in update_files to prevent the file panel from getting stuck showing "Fetching changes...".
• Fix misleading "clipboard" message for copy_hash (uses unnamed reg).
• Fix Windows cmd /c start empty title arg for open_commit_in_browser.
• Guard against empty resolved layout list in cycle_layout to prevent division-by-zero when config contains only unknown layout names.

[Copilot]

Pull request overview

This pull request addresses four distinct bug fixes in the diffview plugin:

Changes:

• Fixed misleading message in copy_hash function (changed "clipboard" to "default register")
• Fixed Windows cmd start command empty title argument
• Added loading state cleanup on error to prevent stuck UI
• Added guard against division-by-zero in cycle_layout when config contains only unknown layout names

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
lua/diffview/scene/views/file_history/listeners.lua	Fixed message accuracy for copy_hash (uses default register not clipboard) and corrected Windows start command syntax
lua/diffview/scene/views/diff/diff_view.lua	Added loading state cleanup in error path to prevent file panel from getting stuck showing "Fetching changes..."
lua/diffview/actions.lua	Refactored layout resolution to prevent division-by-zero when cycling layouts with empty resolved layout list

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