refactor: add error boundaries, improve error handling

[robsimmons]

This PR adds normal Next.js error boundaries for handling exceptional, "talk to your administrator" control flows to the application, simplifies some of the logic around encountering those cases (they'll just throw to the error boundary), and handles many cases where exceptions were being ignored.

Distinguishing normal errors and unexpected exceptions

When I was exploring adding new linting rules I observed a (pervasive, common, classic, first line in Next.js docs on error handling) conflation between two different types of server-side errors:

• Expected in-the-course-of-doing-business errors; ultimately you can't do client-side validation as the final check for "does a project with this name exist," so you need the server function to surface and return these errors. ActionResponse is the right shape for this kind of thing.
• Genuinely unexpected "something is misconfigured" errors. ("Uncaught exceptions," the next.js docs call them, but I don't like that, possibly because I still have trauma from using Java in the naughts.)

Using ActionResponse for the latter use case, and the combination of SWR and serverAction access patterns, meant that there were places where e.g. an Error was getting as-cast to string and then getting placed in the React document like a string, resulting in an error.

Surfacing errors

This PR addresses quite a number of places where uncaught exceptions are thrown in handlers, and these get ignored completely, or at most with a comment in the console. useThrowingSWR and useThrowToBoundary are introduced as patterns for taking a computation outside of the render phase and catching an error for the purpose of re-throwing it to an error boundary.

Additional

• Documents conventions in AGENTS for the agents and the humans like me who read AGENTS.
• Renames the Error component to ErrorBox so it doesn't conflate with built-in Error.
• Fixes a kind-of-security issue with untrusted error searchParams being authoritatively communicated to users

[Vtec234]

Thank you, this seems like an improvement.