opencode --continue fails with "Unexpected server error" (TUI fetches /session/dummy)

Description

On opencode --continue the TUI seeds a placeholder session route with sessionID: "dummy" (packages/tui/src/app.tsx, introduced in fe0c4f8c74 / #31049). The session route eagerly calls session.get({ sessionID }, { throwOnError: true }) on mount, so the client sends GET /session/dummy. The server rejects it with 400 because SessionID requires the ses_ prefix (packages/schema/src/session-id.ts), and the TUI surfaces it as "Unexpected server error. Check server logs for details."

Introduced by #31049 (2026-06-06) which changed the placeholder from undefined to "dummy". Only reproducible when the route mounts before the continue effect resolves the real session id — e.g. OPENCODE_FAST_BOOT=1 or slow /session list.

Steps to reproduce

1. OPENCODE_FAST_BOOT=1 opencode --continue on a machine with session history
2. Watch the server log: GET /session/dummy → 400
3. TUI shows "Unexpected server error"

Plugins

none

OpenCode version

1.18.29 (and dev HEAD bbd72fb8b0)