[FEATURE] Add "Expand All" / "Collapse All" buttons for diffs in the desktop app

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

In the redesigned Claude Code desktop app (released 2026-04-14), each file diff in the chat transcript can be expanded or collapsed individually. When a session has modified many files, this becomes tedious - I have to click every single diff one at a time to either expand all of them to review the full changeset end-to-end, or collapse all of them to scroll back to earlier messages without wading through hundreds of diff lines.

For sessions that touch 10+ files (common for refactors or plan-mode implementations), this is a lot of clicking. I checked the current shortcut list (Cmd+/) and the keybindings docs - the only diff-related shortcut is Cmd+Shift+D, which toggles the diff pane's visibility, not the individual file diffs. There's no bulk action and no per-diff keybinding either.

Proposed Solution

Add two controls to the chat/diff view — likely near the session header or as a floating control in the diff area:

• Expand All - opens every collapsed file diff in the current session
• Collapse All - closes every open file diff in the current session

Exposing them as keybinding actions (e.g., diff:expandAll / diff:collapseAll) in the DiffDialog context would also let power users rebind them via ~/.claude/keybindings.json, consistent with the rest of the keybindings system.

Alternative Solutions

• A "Collapse diffs by default" setting (already proposed in #37584) combined with the existing per-diff click. Less powerful than explicit bulk controls but easier to implement.
• A single toggle button that remembers the last bulk state (expand - collapse - expand) instead of two separate controls.
• Scoping the bulk action to the current message/turn rather than the whole session, useful for reviewing one agent response at a time.

Priority

Medium - Would be very helpful

Feature Category

Other

Use Case Example

I'm working on a Java refactor touching ~20 files across several packages. Claude finishes the work, I scroll up to review, and every file diff is expanded inline. To compare logic across files I want them all expanded - fine. But once I've reviewed, I want to collapse everything so I can write a follow-up prompt with the diffs out of the way. Today I click 20 chevrons one at a time. With "Collapse All" it's one click. Same story in reverse when I re-open a long session and want to re-read what happened - "Expand All" saves me from opening each file diff individually.

Additional Context

• Related existing requests: #37584 (collapse-by-default setting), #17043 (parent thread on diff visual noise in long sessions)
• The existing Cmd+Shift+D shortcut already targets "diff" but only toggles the pane - a good namespace to extend
• Affects any Code-tab session with multiple file edits
• Platform: macOS, Claude Code desktop (new UI, 2026-04-14 release)