Add Super Groups: semi-public symmetrically-encrypted groups where all members can send

[Copilot]

• Previous session: implemented Super Groups with OutSuperGroup/InSuperGroup split (incorrect approach)
• Replace OutSuperGroup = 170 and InSuperGroup = 175 with single SuperGroup = 170 in src/constants.rs
• Add Param::SuperGroupAdmin to src/param.rs to store admin contact ID
• Update create_out_super_group_ex() in src/chat.rs to use SuperGroup type and store ContactId::SELF as admin
• Add Chat::is_self_super_group_admin() and Chat::get_super_group_admin_id() methods
• Replace all OutSuperGroup/InSuperGroup references with SuperGroup in src/chat.rs
• Update is_self_in_chat() to use contacts table for SuperGroup (like Group)
• Update block_ex() - SuperGroup admin can't block it
• Update src/mimefactory.rs for single SuperGroup type
• Update src/receive_imf.rs - merge apply_out_super_group_changes + apply_in_super_group_changes into apply_super_group_changes, store admin from from_id
• Update src/securejoin/bob.rs - create SuperGroup instead of InSuperGroup, store inviter as admin
• Update src/securejoin.rs, src/securejoin/qrinvite.rs, src/qr.rs, src/message.rs, src/stats.rs
• Build and verify

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