Challenge mode system

[benSlaughter]

The "Challenges" card on the home page says "Coming Soon". Time to build it.

What: Weekly themed challenges that encourage playing across different game types.

Examples:

• "The 7× Table Challenge" — get 10 correct answers on 7× table across any maths game
• "Spelling Sprint" — practise all words in the active list using at least 3 different games
• "Explorer Week" — play every game type at least once

Data model:

CREATE TABLE challenges (
  id INTEGER PRIMARY KEY,
  title TEXT, description TEXT,
  challenge_type TEXT,  -- 'maths_table', 'spelling_all', 'play_all'
  target_value TEXT,    -- '7' for 7× table, etc.
  target_count INTEGER, -- how many to complete
  active_from DATE, active_until DATE
);
CREATE TABLE challenge_progress (
  challenge_id INTEGER, profile_id INTEGER,
  current_count INTEGER DEFAULT 0,
  completed_at DATETIME
);

Admin: Add a "Challenges" section to set active challenges.
Reward: Special challenge badges, distinct from regular achievements.