Add student delight features: celebrations, sounds, animations, and background patterns

- Confetti + pop sound on lesson/assignment completion
- Done stamp animation (awaitable, shows below toast)
- Welcome toast on first login only (not back-navigation)
- Web Audio ding/buzz during spelling practice and flashcards
- Fanfare on all-day completion
- Score counter animate-up on practice/test results
- 3D flashcard flip animation (rotateY via CSS keyframes)
- Tab slide transition (Today/Courses/Spelling, directional)
- Staggered card entrance + time-of-day greeting on home screen
- Emoji wobble on kid card hover
- Word card green/red flash on correct/wrong spelling answer
- Combo streak celebration at milestones (3, 5, 7, 10 in a row)
- Random cheer messages for generic celebrations
- Button bounce on :active (scale + translateY)
- Progress bar smooth transition (0.5s ease-out)
- Background pattern picker: dots, stripes, grid, stars, bubbles
- bg_pattern persisted per-student via PATCH /api/students/:id/bg-pattern
- Theme switching preserves bg pattern class

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

db.js

@@ -228,6 +228,7 @@ tryAlter(`ALTER TABLE students ADD COLUMN theme TEXT NOT NULL DEFAULT 'blue'`);
 tryAlter(`ALTER TABLE students ADD COLUMN streak_date TEXT`);
 tryAlter(`ALTER TABLE students ADD COLUMN streak_count INTEGER NOT NULL DEFAULT 0`);
 tryAlter(`ALTER TABLE students ADD COLUMN best_streak INTEGER NOT NULL DEFAULT 0`);
+tryAlter(`ALTER TABLE students ADD COLUMN bg_pattern TEXT NOT NULL DEFAULT 'none'`);
 tryAlter(`ALTER TABLE lists ADD COLUMN group_name TEXT NOT NULL DEFAULT ''`);
 tryAlter(`ALTER TABLE decks ADD COLUMN group_name TEXT NOT NULL DEFAULT ''`);