feat: add theme_mode, theme_light, theme_dark to UserAppearanceSettings (#25076)

Part 1: Backend portion of a change broken into 2 PRs.
Part 2: #25077 

Adds three new UserAppearanceSettings fields (theme_mode, theme_light,
theme_dark) on top of the existing theme_preference and terminal_font.
Replaces GetUserThemePreference and GetUserTerminalFont with a single
GetUserAppearanceSettings aggregate query. The PUT handler is wrapped in
db.InTx so sync-mode's mode + slot writes can never half-apply.

site/src/api/queries/users.ts

@@ -289,6 +289,9 @@ export const updateAppearanceSettings = (
 			// more responsive.
 			queryClient.setQueryData(myAppearanceKey, {
 				theme_preference: patch.theme_preference,
+				theme_mode: patch.theme_mode,
+				theme_light: patch.theme_light,
+				theme_dark: patch.theme_dark,
 				terminal_font: patch.terminal_font,
 			});
 		},

site/src/api/typesGenerated.ts

@@ -7908,6 +7908,11 @@ export const TerminalFontNames: TerminalFontName[] = [
 	"",
 ];
 
+// From codersdk/users.go
+export type ThemeMode = "single" | "sync" | "";
+
+export const ThemeModes: ThemeMode[] = ["single", "sync", ""];
+
 // From codersdk/users.go
 export type ThinkingDisplayMode =
 	| "always_collapsed"
@@ -8398,6 +8403,28 @@ export interface UpdateTemplateMeta {
 // From codersdk/users.go
 export interface UpdateUserAppearanceSettingsRequest {
 	readonly theme_preference: string;
+	/**
+	 * ThemeMode is optional for backward compatibility. When empty,
+	 * the server leaves theme_mode, theme_light, and theme_dark
+	 * unchanged so older CLI clients do not erase sync-mode settings.
+	 * Legacy auto preferences are the exception: they clear theme_mode
+	 * so clients can migrate the old sync-with-system setting.
+	 */
+	readonly theme_mode: ThemeMode;
+	/**
+	 * ThemeLight is required when ThemeMode is "sync". In "single"
+	 * mode an empty value means "preserve the previously persisted
+	 * slot" rather than "clear the slot", so partial updates that send
+	 * only one slot keep the other intact.
+	 */
+	readonly theme_light: string;
+	/**
+	 * ThemeDark is required when ThemeMode is "sync". In "single" mode
+	 * an empty value means "preserve the previously persisted slot"
+	 * rather than "clear the slot", so partial updates that send only
+	 * one slot keep the other intact.
+	 */
+	readonly theme_dark: string;
 	readonly terminal_font: TerminalFontName;
 }
 
@@ -8699,7 +8726,24 @@ export interface UserActivityInsightsResponse {
 
 // From codersdk/users.go
 export interface UserAppearanceSettings {
+	/**
+	 * ThemePreference is the legacy single-field appearance setting. In
+	 * "single" mode it mirrors the active theme. In "sync" mode modern
+	 * clients normally mirror the active OS slot, but older clients can
+	 * update only this field, so it may diverge from ThemeLight or
+	 * ThemeDark until a modern client saves the full appearance state
+	 * again.
+	 */
 	readonly theme_preference: string;
+	readonly theme_mode: ThemeMode;
+	/**
+	 * Ignored when ThemeMode is "single"
+	 */
+	readonly theme_light: string;
+	/**
+	 * Ignored when ThemeMode is "single"
+	 */
+	readonly theme_dark: string;
 	readonly terminal_font: TerminalFontName;
 }