Accounts for kids with parent approval

What changed

• Children under 13 can now make an account with a parent's approval. Sign-up asks for birth month and year, and these accounts stay inside AlgoArena.
• You can rate replies from Rena and the other AI helpers, with an optional reason. Rena's code proposals show as diffs in the editor, and Rena sits beside submission and match reviews.
• Mock interviews use their own interviewer personas, separate from Rena. The font size setting scales the whole page, and when code cannot run, you see a plain message.