[FEATURE] No theme token for the diff panel background

### Preflight Checklist

- [x] I have searched [existing requests](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20label%3Aenhancement) and this feature hasn't been requested yet
- [x] This is a single feature request (not multiple features)

### Problem Statement

### Summary

The diff panel paints its own background, and no theme token controls it. On a dark base it
fills with a fixed dark neutral that has no relation to the active theme or to the
terminal's background.

### Detail

The color token reference in `terminal-config` covers added/removed line colors
(`diffAdded`, `diffRemoved`, the `*Dimmed` and `*Word` variants) and three transcript
message backgrounds (`userMessageBackground`, `bashMessageBackgroundColor`,
`memoryBackgroundColor`), but nothing for the diff panel's own canvas. Grepping the 2.1.272
binary for `background`, `panelBackground`, `secondaryBackground`, `diffBackground`, and
`canvas` turns up no such token either.

Confirmed by capturing what the pane actually emits (`tmux capture-pane -p -e`): the panel
fill is an explicit background color on ~149 cells, distinct from the diff line colors and
distinct from `49` (terminal default). Cells the panel doesn't paint fall through to the
terminal default correctly — it's only this explicitly filled region that can't be themed.

The result: on a terminal with a tinted background (e.g. Dracula's `#282a36`), the panel
reads as a near-black rectangle inside an otherwise themed UI, and no custom theme can
change it.

### Request

Expose a token for the diff panel background, or leave those cells at the terminal default
(`49`) so they inherit the terminal's background like the rest of the transcript.

### Environment

- Claude Code 2.1.272, macOS 15 (Darwin 25.5.0), tmux 3.6b, Alacritty

### Proposed Solution

should be configurable in theme settings

### Alternative Solutions

none.

### Priority

Low - Nice to have

### Feature Category

Interactive mode (TUI)

### Use Case Example

_No response_

### Additional Context

_No response_

Start by locating the terminal-config theme token definitions and the diff panel rendering path. Compare the panel's explicit background fill with the existing diff and transcript background tokens. Done means the panel background is configurable through theme settings or inherits the terminal default without changing the existing diff colors.

Written by the indexing model from the issue text.