Add theming infrastructure (task 08)

CSS-variable multi-theme system (default + warm) × {light, dark, system}; per-user theme/theme_mode columns; no-flash pre-paint script; useTheme hook + ThemePicker component on /settings.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/modules/_core/themes.ts

@@ -0,0 +1,16 @@
+export type ThemeMode = "light" | "dark" | "system";
+export type ThemeId = "default" | "warm" | (string & {});
+
+export const THEMES: ReadonlyArray<{ id: ThemeId; label: string }> = [
+  { id: "default", label: "Default" },
+  { id: "warm", label: "Warm" },
+];
+
+export const THEME_MODES: ReadonlyArray<{ id: ThemeMode; label: string }> = [
+  { id: "light", label: "Light" },
+  { id: "dark", label: "Dark" },
+  { id: "system", label: "System" },
+];
+
+export const VALID_THEME_IDS = new Set(THEMES.map((t) => t.id));
+export const VALID_THEME_MODES = new Set<string>(["light", "dark", "system"]);