Add validation to prevent joining non-existent groups

[Copilot]

Addresses missing validation in handleJoinGroup that allowed users to join non-existent group codes, creating invalid empty group entries.

Changes

• Group existence check: Validate group exists in database before allowing join
• Group validity check: Verify group has either meta information or at least one member
• Error handling: Return early with descriptive alerts on validation failure

// Validate that the group exists before joining
try {
    const groupSnap = await database.ref(`groups/${code}`).once('value');
    if (!groupSnap.exists()) {
        alert('Group code does not exist. Please check the code or create a new group.');
        return;
    }
    
    // Check if group has meta information or at least one member
    const metaSnap = await database.ref(`groups/${code}/meta`).once('value');
    const membersSnap = await database.ref(`groups/${code}/members`).once('value');
    
    if (!metaSnap.exists() && !membersSnap.exists()) {
        alert('This group appears to be invalid. Please create a new group or use a different code.');
        return;
    }
} catch (e) {
    console.error('Error validating group:', e);
    alert('Error validating group. Please try again.');
    return;
}

Fixes #1 (comment)

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