Expanded changes view, organization settings, and diff improvements

Expanded Changes Sidebar #858

• New workspace layout with a resizable sidebar and improved expand/collapse behavior
• Main content switches cleanly between tab view and changes view
• Added a placeholder "changes" view for upcoming code-review support

Organization Settings #820

• Desktop: pick an image file to set organization logo (preview + upload)
• Organization settings: edit name, manage slug with live availability checks (owner-only)
• Members: invite, view, sort, and manage organization members with role-aware actions
• Settings navigation reorganized from team-centric to separate Organization and Members sections

Diff View Improvements #845

• Added ability to toggle visibility of unchanged regions in the diff viewer
• Users can now hide unchanged lines to focus on actual changes, improving readability of large diffs
• New toggle button added to the file viewer toolbar with visual state indication

Middle-Click to Close Tabs #853

Added middle-click support for tabs—users can now close a tab by middle-clicking on it for quicker navigation. Matches standard browser tab behavior.