fix: respect light/dark theme in agents FilesChangedPanel diff viewer (#22403)

## Problem

The git diff on the `/agents` page had color issues: the editor
background followed light mode but the syntax highlighting used dark
mode (`github-dark-high-contrast`), and the filename header used
light-colored text on a light background.

The root cause was hardcoded dark theme options in the `FileDiff`
component:

```tsx
themeType: "dark",
theme: "github-dark-high-contrast",
```

## Fix

Uses the same theme-aware pattern as every other diff/file viewer in the
codebase (`WriteFileTool`, `EditFilesTool`, `ReadFileTool`, `Tool`,
`response.tsx`):

1. `useTheme()` from `@emotion/react` to read `palette.mode`
2. `getDiffViewerOptions(isDark)` from the shared `utils.ts` module —
returns `github-light` theme for light mode, `github-dark-high-contrast`
for dark mode
3. Reuses `DIFFS_FONT_STYLE` and `diffViewerCSS` constants instead of
inlining duplicates

## Storybook coverage

Added four new stories with real unified diff content:
- **WithDiffDark** — dark mode with a PR link
- **WithDiffLight** — light mode with a PR link
- **NoPullRequestDark** — dark mode, "Files Changed" header
- **NoPullRequestLight** — light mode, "Files Changed" header

The existing stories only covered empty and parse-error states with no
rendered diff.

site/src/pages/AgentsPage/FilesChangedPanel.stories.tsx

@@ -5,6 +5,46 @@ import type { ChatDiffContents } from "api/typesGenerated";
 import { expect, screen, spyOn } from "storybook/test";
 import { FilesChangedPanel } from "./FilesChangedPanel";
 
+const sampleUnifiedDiff = `diff --git a/site/src/pages/AgentsPage/FilesChangedPanel.tsx b/site/src/pages/AgentsPage/FilesChangedPanel.tsx
+index abc1234..def5678 100644
+--- a/site/src/pages/AgentsPage/FilesChangedPanel.tsx
++++ b/site/src/pages/AgentsPage/FilesChangedPanel.tsx
+@@ -1,10 +1,14 @@
++import { useTheme } from "@emotion/react";
+ import { parsePatchFiles } from "@pierre/diffs";
+ import { FileDiff } from "@pierre/diffs/react";
++import {
++  DIFFS_FONT_STYLE,
++  getDiffViewerOptions,
++} from "components/ai-elements/tool/utils";
+ import { chatDiffContents, chatDiffStatus } from "api/queries/chats";
+ import { ErrorAlert } from "components/Alert/ErrorAlert";
+ import { ScrollArea } from "components/ScrollArea/ScrollArea";
+-import { Skeleton } from "components/Skeleton/Skeleton";
+ import { type FC, useMemo } from "react";
+ import { useQuery } from "react-query";
+diff --git a/site/src/components/ai-elements/tool/utils.ts b/site/src/components/ai-elements/tool/utils.ts
+index 1234567..abcdef0 100644
+--- a/site/src/components/ai-elements/tool/utils.ts
++++ b/site/src/components/ai-elements/tool/utils.ts
+@@ -10,6 +10,18 @@ export const diffViewerCSS =
+ export function getDiffViewerOptions(isDark: boolean) {
+   return {
+     diffStyle: "unified" as const,
++    diffIndicators: "bars" as const,
++    overflow: "scroll" as const,
+     themeType: (isDark ? "dark" : "light") as "dark" | "light",
++    theme: isDark ? "github-dark-high-contrast" : "github-light",
++    unsafeCSS: diffViewerCSS,
+   };
+ }
++
++export const DIFFS_FONT_STYLE = {
++  "--diffs-font-size": "11px",
++  "--diffs-line-height": "1.5",
++} as React.CSSProperties;
+`;
+
 const defaultDiffStatus: ChatDiffStatusResponse = {
 	chat_id: "test-chat",
 	changes_requested: false,
@@ -23,6 +63,13 @@ const meta: Meta<typeof FilesChangedPanel> = {
 	args: {
 		chatId: "test-chat",
 	},
+	decorators: [
+		(Story) => (
+			<div style={{ height: 600, width: 500 }}>
+				<Story />
+			</div>
+		),
+	],
 	beforeEach: () => {
 		spyOn(API, "getChatDiffStatus").mockResolvedValue(defaultDiffStatus);
 		spyOn(API, "getChatDiffContents").mockResolvedValue(defaultDiffContents);
@@ -65,3 +112,73 @@ export const ParseError: Story = {
 		expect(screen.getByText("No file changes to display.")).toBeInTheDocument();
 	},
 };
+
+export const WithDiffDark: Story = {
+	beforeEach: () => {
+		spyOn(API, "getChatDiffStatus").mockResolvedValue({
+			...defaultDiffStatus,
+			url: "https://github.com/coder/coder/pull/456",
+			additions: 14,
+			deletions: 2,
+			changed_files: 2,
+		});
+		spyOn(API, "getChatDiffContents").mockResolvedValue({
+			...defaultDiffContents,
+			diff: sampleUnifiedDiff,
+		});
+	},
+};
+
+export const WithDiffLight: Story = {
+	globals: {
+		theme: "light",
+	},
+	beforeEach: () => {
+		spyOn(API, "getChatDiffStatus").mockResolvedValue({
+			...defaultDiffStatus,
+			url: "https://github.com/coder/coder/pull/456",
+			additions: 14,
+			deletions: 2,
+			changed_files: 2,
+		});
+		spyOn(API, "getChatDiffContents").mockResolvedValue({
+			...defaultDiffContents,
+			diff: sampleUnifiedDiff,
+		});
+	},
+};
+
+export const NoPullRequestDark: Story = {
+	beforeEach: () => {
+		spyOn(API, "getChatDiffStatus").mockResolvedValue({
+			...defaultDiffStatus,
+			url: "https://github.com/coder/coder/pull/456",
+			additions: 14,
+			deletions: 2,
+			changed_files: 2,
+		});
+		spyOn(API, "getChatDiffContents").mockResolvedValue({
+			...defaultDiffContents,
+			diff: sampleUnifiedDiff,
+		});
+	},
+};
+
+export const NoPullRequestLight: Story = {
+	globals: {
+		theme: "light",
+	},
+	beforeEach: () => {
+		spyOn(API, "getChatDiffStatus").mockResolvedValue({
+			...defaultDiffStatus,
+			url: "https://github.com/coder/coder/pull/456",
+			additions: 14,
+			deletions: 2,
+			changed_files: 2,
+		});
+		spyOn(API, "getChatDiffContents").mockResolvedValue({
+			...defaultDiffContents,
+			diff: sampleUnifiedDiff,
+		});
+	},
+};

site/src/pages/AgentsPage/FilesChangedPanel.tsx

@@ -1,7 +1,12 @@
+import { useTheme } from "@emotion/react";
 import { parsePatchFiles } from "@pierre/diffs";
 import { FileDiff } from "@pierre/diffs/react";
 import { chatDiffContents, chatDiffStatus } from "api/queries/chats";
 import { ErrorAlert } from "components/Alert/ErrorAlert";
+import {
+	DIFFS_FONT_STYLE,
+	getDiffViewerOptions,
+} from "components/ai-elements/tool/utils";
 import { ScrollArea } from "components/ScrollArea/ScrollArea";
 import { Skeleton } from "components/Skeleton/Skeleton";
 import {
@@ -33,6 +38,10 @@ function formatPullRequestLabel(url: string): string {
 }
 
 export const FilesChangedPanel: FC<FilesChangedPanelProps> = ({ chatId }) => {
+	const theme = useTheme();
+	const isDark = theme.palette.mode === "dark";
+	const diffOptions = useMemo(() => getDiffViewerOptions(isDark), [isDark]);
+
 	const diffStatusQuery = useQuery(chatDiffStatus(chatId));
 	const diffContentsQuery = useQuery({
 		...chatDiffContents(chatId),
@@ -134,23 +143,14 @@ export const FilesChangedPanel: FC<FilesChangedPanelProps> = ({ chatId }) => {
 								key={fileDiff.name}
 								fileDiff={fileDiff}
 								options={{
-									diffStyle: "unified",
-									diffIndicators: "bars",
-									overflow: "scroll",
-									themeType: "dark",
+									...diffOptions,
 									enableLineSelection: true,
 									enableHoverUtility: true,
 									onLineSelected() {
 										// TODO: Make this add context to the input so the user can type.
 									},
-									theme: "github-dark-high-contrast",
-									unsafeCSS:
-										"pre, [data-line], [data-diffs-header] { background-color: transparent !important; } [data-diffs-header] { border-left: 1px solid var(--border); }",
-								}}
-								style={{
-									"--diffs-font-size": "11px",
-									"--diffs-line-height": "1.5",
 								}}
+								style={DIFFS_FONT_STYLE}
 							/>
 						))}
 					</div>