feat: display token usage and cost in session detail view

[bencered]

The bridge already sends step_finish SSE events containing cost and tokens data from opencode, but the frontend currently ignores them. We should display cumulative token usage and cost in the session detail view.

Current State

• bridge.py emits step_finish events with cost (dollar amount) and tokens (input/output counts)
• The frontend (session/[id]/page.tsx) has no rendering for step_finish events
• No usage summary anywhere in the session UI

Proposed

• Track cumulative tokens (input + output) and cost from step_finish events
• Display a usage summary bar in the session detail view (e.g. below the header or in the right sidebar)
• Show per-message token counts inline (subtle, like a tooltip or small badge)
• Update in real-time as new step_finish events arrive via SSE

[ColeMurray]

#416