Add TypeCheck to this server button when the bot is missing

[CompleteDotTech]

Problem

After #56, launching the Activity in a server without the bot shows "The TypeCheck bot is not in this server yet…", telling the member to open TypeCheck in a browser. Inside Discord the homepage Add to your server button is hidden, and Connect in Discord only signs in, so clicking it just repeats the message. There was no way to add the bot from where the member was.

Change

• Only when sign-in returns bot_not_in_server, the error shows an Add TypeCheck to this server button. It calls the SDK's openExternalLink with Discord's install URL, with the launching server preselected (guild_id + disable_guild_select=true).
• A note under the button: adding the bot requires the Manage Server permission (server owners and Administrators have it).
• The message now says that Connect in Discord only signs you in and cannot add the bot.
• The homepage and Activity build the install link from one installUrl() helper (permissions 1374389611526).

Once the bot is in the server, sign-in no longer returns bot_not_in_server, so the button does not appear.

Validation

• npm run check clean; npm test 383/383 pass, including tests for the error type and the install URL (with and without a preselected server).
• SHA256SUMS refreshed and verified.
• Not verified inside the Discord client: the button's openExternalLink behavior needs a launch in a server without the bot.

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