School mode: username + kid-friendly password login

[benSlaughter]

Support many children (a whole class) with simple, child-friendly authentication.

This is Mode 3 (School) of the 3-tier profile system. See docs/ROADMAP.md.

How it works

• Admin sets profile_mode to school in settings
• On app launch, a login screen asks for username + password
• Login is designed for 10-year-olds — not a corporate form

Kid-friendly password options (pick one or offer choice per child)

1. Colour code — pick 4 colours from a grid (e.g. red-blue-green-yellow)
2. Picture password — tap 4 icons in order from a grid of 12
3. Short PIN — 4-digit number pad (simple, familiar)
4. Text password — traditional, for older/confident kids

Whichever method, keep it visual, big buttons, no keyboard required if possible.

Username

• Set by admin (first name, or nickname)
• Shown on login screen as a list or typed in (depending on class size)
• For small groups: show avatar cards like family mode, but require password after tapping
• For large groups: type-to-search username list

Admin features

• Create/edit/delete student profiles (name, avatar, password method)
• Reset password for any student
• View all students' progress in a table/grid
• Bulk import students (CSV: name, optional username)
• Optional: group students by class/year

Database changes

• Add to profiles: username (unique), password_hash, password_type (colour/picture/pin/text)
• Add settings key: profile_mode = school
• Session management: student sessions (separate from admin sessions)

Security notes

• Student passwords are hashed (bcrypt) same as admin
• Student sessions are cookies, same pattern as admin auth
• No email, no personal data beyond first name
• Admin can see/reset passwords but never view them

Prerequisites

• Family mode: multiple profiles without passwords #21 (family mode) — builds the multi-profile foundation
• Character avatar system #14 (character avatars) — avatar selection for profiles