fix(app): remove background running indicator

[opencode-agent]

Summary

• Remove the extra Running label and spinner below the transcript.
• Leave the existing Working condition and presentation unchanged: an idle session with background shell/subagent work shows no bottom-of-transcript busy indicator.
• Keep tool/subagent indicators and background-work controls unchanged; remove the unused English key and import.

Validation

• App typecheck and E2E typecheck pass.
• Production app build and scoped formatting check pass.
• All 14 existing session-timeline-working.spec.ts browser tests passed during validation. The added regression cases were removed at the requester's direction; the test file is identical to the base and no test changes are included in this PR. E2E typecheck and formatting passed again after removal.
• Production warm-tab benchmark smoke passes on both refs (one sample each, not a performance distribution): first-correct observation 56.8 → 67.3 ms; stable observation 78.8 → 95.6 ms.
• Browser/build evidence compared base 4680a4aa6f84f4f2680b267b0607b26218fb921b with 1bda2831578cfd1a233aef9d52a09b56cda17510, using Chromium 147 on Linux and deterministic mocked backend data. The subsequent update only removes the added tests; production code is unchanged. Browser checks use Node 22; the host's Node 18 could not run this harness correctly.

Requested by: @Hona (Hona via Slack)