fix(tui): recover to Home when the session route fails to load

[YOMXXX]

Summary

Fixes #1936. A malformed --session/-s value (e.g. ses_…、) left the TUI on a black screen instead of showing an error and returning to Home.

Root cause: the session-route loader (packages/opencode/src/cli/cmd/tui/routes/session/index.tsx createEffect) calls sdk.client.session.get(…, { throwOnError: true }) and session.actors(…, { throwOnError: true }). With throwOnError, an HTTP error rejects rather than resolving { data: undefined }. The loader had no catch boundary, so a rejection escaped the async effect with the route still set to the failed session → black screen. The same missing boundary underlies the --continue "dummy" placeholder case (#1809).

Fix: wrap the loader body in try/catch. On any load failure, log the error, show a Cannot open session toast, and navigate Home — matching the existing Session not found / !verdict.renderable recovery behavior. No session-ID grammar rewriting required.

Test Plan

• bun typecheck — clean.
• Manual (maintainer): mimo -s 'ses_bogus、' → error toast + navigate Home, no black screen.

Notes

• No unit test: the SolidJS route loader isn't unit-testable in the repo's current TUI test infra; verified by typecheck + code review.
• Related: Bug Report: mimo -c (continue) shows black screen after upgrade — sessionID "dummy" rejected by backend #1809 (same missing recovery boundary via the --continue placeholder) benefits from the same catch.