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/schema.ts

@@ -19,6 +19,8 @@ export const users = pgTable("users", {
   email: varchar("email", { length: 255 }).notNull().unique(),
   emailVerified: timestamp("email_verified", { withTimezone: true }),
   image: text("image"),
+  theme: text("theme").notNull().default("default"),
+  themeMode: text("theme_mode").notNull().default("system"),
   createdAt: timestamp("created_at", { withTimezone: true }).notNull().defaultNow(),
 });