The app page's run log and tail update from /events, not their own EventSources #2844

Split from #2835 (parked twice). Both pulls before this failed test/map.test.ts › "the drawing is a live region scoped to its repository, fits, and pans". That test asserts the literal new EventSource('/events') in the shared src/web/client.ts, so leave connect() as it is. Run bun test test/map.test.ts before you push.

Done looks like:

• [ ] The app page's deploy/run log tail (the new EventSource(url) near src/web/client.ts:436) appends lines from run.log frames over the one /events connection.
• [ ] The mirror progress stream (client.ts:126) is left alone. It is not a run log, and moving it is a separate change.
• [ ] Test: the app page opens no run-log EventSource besides /events, and test/map.test.ts passes unchanged.

Files: src/web/client.ts, the app page module under src/web/, a test under test/.

Parked by the crew: the check failed and the decider saw no point in another try (pull #2849 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.

Parked by the crew: the check failed and the decider saw no point in another try (pull #2850 closed).

Parked twice since a person last spoke here, so it is the planner's now. Every reason:

1. the check failed and the decider saw no point in another try (pull #2849 closed)
2. the check failed and the decider saw no point in another try (pull #2850 closed)

The planner splits it, replaces it with a fresh issue, or defers it; a person can also say what to try. Nothing starts here until then, and the board lists it.

Replaced by #2854. Both pulls (#2849, #2850) failed only the type check, on the same line of their own new test: test/app-tail-events.test.ts(36,5): error TS2722: Cannot invoke an object which is possibly 'undefined'. #2854 names that error and how to guard it, notes that the run.log frame is now on main (#2845), and asks for bun x tsc --noEmit before the push.