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>

drizzle/0002_naive_starbolt.sql

@@ -0,0 +1,2 @@
+ALTER TABLE "users" ADD COLUMN "theme" text DEFAULT 'default' NOT NULL;--> statement-breakpoint
+ALTER TABLE "users" ADD COLUMN "theme_mode" text DEFAULT 'system' NOT NULL;