[FEATURE] Desktop: option to disable or adjust dimming of the unfocused pane in split view

### Preflight Checklist

- [x] I have searched [existing requests](https://github.com/anthropics/claude-code/issues?q=is%3Aissue+label%3Aenhancement) and this feature hasn't been requested yet
- [x] This is a single feature request (not multiple features)

### Problem Statement

In the desktop app, when two Code sessions are open side by side (split view), the pane that is not focused is dimmed. There is no setting to turn this off or adjust it, and the dimming makes the unfocused side noticeably harder to read — when I need to read or compare content on the other side, I have to click into it first just to make it clearer.

The effect appears to be applied as a CSS filter on the unfocused pane (roughly `opacity(.75) saturate(.85)`), and it is not exposed anywhere in Settings.

### Proposed Solution

Add a setting to control it, e.g. under Settings → Claude Code:
- A simple toggle: "Dim unfocused panes" (on by default, matching today's behavior), or
- A few intensity levels (Off / Subtle / Default)

Keeping today's dimming as the default is fine; the request is just to let users turn it off or lower it.

### Steps to Reproduce

1. In the desktop app (Code tab), open a session.
2. Drag a second session from the sidebar to open it side by side (split view).
3. The unfocused pane renders dimmed; clicking the other pane moves the dimming to the other side.

### Environment

- Claude Desktop 1.52386.6 on Windows 11
- Happens in both light and dark themes

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🤖 Drafted and filed by Claude (Claude Code) at the user's request.

Start in the desktop app's Code tab and Settings → Claude Code, then locate the CSS filter applied to the unfocused split-view pane. Decide whether the setting is a toggle or intensity levels, preserve the current dimming by default, and verify that the selected setting controls the pane in both light and dark themes.

Written by the indexing model from the issue text.