chore: add plan files for Phase 3 advanced features

plans/phase-3-advanced-features/phase-3-user-themes.md

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+# Phase 3: User Theme Overrides
+
+**Status:** ⬜ Not Started
+**Parent plan:** [PLAN.md](./PLAN.md)
+**Domain:** fullstack
+
+## Tasks
+- [ ] Task 1: Add `themeMode`, `primaryHue`, `primarySaturation`, `backgroundType`, `locale` fields to User model (Prisma migration)
+- [ ] Task 2: Create `src/routes/api/users/me/preferences/+server.ts` — GET/PATCH user preferences
+- [ ] Task 3: Create `src/routes/settings/+page.server.ts` — user settings page data
+- [ ] Task 4: Create `src/routes/settings/+page.svelte` — user settings page with theme customization
+- [ ] Task 5: Create `src/lib/components/settings/ThemeCustomizer.svelte` — HSL color picker, background selector, mode toggle
+- [ ] Task 6: Update theme store to load user preferences from server on login
+- [ ] Task 7: Update `+layout.server.ts` to pass user preferences
+- [ ] Task 8: Add user settings link to header user menu
+- [ ] Task 9: Add i18n translations (EN/RU)
+
+## Handoff to Next Phase
+<!-- Filled in after completion -->