Desktop app: open files in tabs instead of a single replacing side panel

Problem

Reviewing a change across several files during one task is impractical: each newly opened file
replaces the previous one in the side panel. There is no way to keep two files open, switch
between them, or return to one after following a link, so the reader loses their place constantly.

Desired behavior

Files open as tabs in the viewer, the way OpenAI Codex does it: several files open at once,
switching between them, and a tab stays until it is closed. A split view for comparing two files
would cover the remaining case.

Acceptance criteria

• Opening a second file does not close the first.
• The user can switch between open files and close them individually.
• Reopening an already-open file focuses its tab instead of creating a duplicate.

Environment

macOS (Darwin 25.6.0), Claude desktop app 1.44121.4, Claude Code 2.1.258.