Add admin panel with storage overview and user management

[Copilot]

• Explore codebase structure
• Add is_admin field support to user records in db_auth.py
• Add admin routes to app.py
• Create templates/admin.html with storage stats and user management table
• Add admin link to dashboard for admin users
• Fix register_user bug: initialize 'users'/'files' keys when missing (fresh DB) so update_database.py works on first run
• Fix register_user bug: update existing user record (quota, is_admin, password) instead of appending a duplicate entry — prevents is_admin_user returning False for the pre-existing entry
• Harden check_if_user_exists, is_admin_user, check_if_auth against missing 'users' key

Original prompt

Make an admin panel that lets the administrator user able to see how much storage is being used, and a table of user management.

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[fybedev]

@copilot
I did register_user('admin', 'admin', quota_gb=10, is_admin=True) and logged in with (admin, admin) but it said this:
Access denied. Admin privileges required.

[fybedev]

Admin permission checking doesn't work properly

[Copilot]

@copilot
I did register_user('admin', 'admin', quota_gb=10, is_admin=True) and logged in with (admin, admin) but it said this:
Access denied. Admin privileges required.

Two bugs were causing this (fixed in 1238c31):

1. Fresh database: register_user called l_db['users'].append(...) but the 'users' key didn't exist yet, raising a KeyError — so the admin user was silently never created. register_user now initializes 'users' and 'files' if they're missing.

2. Pre-existing user: If an admin account was created first via the web /register page (without is_admin), calling register_user('admin', ..., is_admin=True) appended a second entry. is_admin_user found the first one (no is_admin) and returned False. register_user now updates the existing user record (quota, is_admin, and password) instead of creating a duplicate.

After the fix, python update_database.py correctly sets up the admin user and /admin is accessible: