Port live timer + token counter for TUI (revive PR #20220 / issue #10739)

[axisrow]

Summary

Port PR anomalyco#20220 ("feat(tui): live per-message response timing") into this fork and rebuild the local binary.

Background

• [FEATURE]: Show a live running timer for prompts in the TUI anomalyco/opencode#10739 ("Show a live running timer for prompts in the TUI") was auto-closed for inactivity after 90 days, without being addressed.
• PR feat(tui): live per-message response timing anomalyco/opencode#20220, which implemented it, was closed by an automated PR-cleanup bot on 2026-05-15 — never reviewed by a maintainer.
• A follow-up comment on the closed PR (2026-06-17) independently asked for the same thing plus a live token counter, referencing Claude's UI.

Upstream isn't going to pick this up on its own; tracking it here to apply locally.

Scope

1. Port the elapsed-time logic from PR feat(tui): live per-message response timing anomalyco/opencode#20220 onto the current packages/opencode/src/cli/cmd/tui/routes/session/index.tsx (the PR's base has since diverged).
2. Extend it with a live token counter (↑ input / ↓ output, using existing step-finish part data), so a frozen counter next to a running timer is a visible signal that the stream died — see kimi-k3 sessions hang indefinitely with no visible indication #1 in this fork.
3. Rebuild the local binary and verify against a real kimi-k3 session.

Acceptance

• While a turn is in progress: elapsed time updates every ~1s, stops on completion.
• Token counts (↑/↓) update as step-finish parts arrive during streaming.
• bun test and tsc --noEmit pass.