Fix gametype filter UI bugs and code quality issues

[bretonium]

• Fix HSplitBottom → HSplitTop so gametype filter header is actually visible
• Simplify token parsing: drop pGametypeNext, use local pToken per iteration
• Replace magic number 9 with MAX_GAMETYPE_FILTERS constant
• Size s_Buttons[MAX_GAMETYPE_FILTERS + 1] (was [11] with unexplained +1)
• Move "No filter" button to s_Buttons[MAX_GAMETYPE_FILTERS] (index 9)
• Rename isSelected → IsSelected to match project convention
• Switch Config()->m_Cl* to g_Config.m_Cl* for consistency with surrounding code
• Add bounds check via minimum() in str_copy for token length
• Document cl_position_community_filter values 0/1/2 in config description
• Remove attribution comment

Checklist

• Tested the change ingame
• Provided screenshots if it is a visual change
• Tested in combination with possibly related configuration options
• Written a unit test (especially base/) or added coverage to integration test
• Considered possible null pointers and out of bounds array indexing
• Changed no physics that affect existing maps
• Tested the change with ASan+UBSan or valgrind's memcheck (optional)