Cost tiers: surface per-turn + per-session cost in the GUI

[jcast90]

Why

Without visible cost, users cannot tell whether a preset is actually working or whether a turn over-spent. We will get bug reports that are really preset complaints. Closing the feedback loop is what makes the cost-tier system trustworthy.

Depends on

• Per-role model routing foundation

Scope

• Per-turn footer: model used, role, input/output tokens, $ estimate
• Channel header: rolling session cost
• Settings → Cost tab: per-day / per-week breakdown by role and model
• Cost data sourced from existing token accounting (do not re-invent metering)

Out of scope

• Hard budget caps / kill switches (track separately if demand emerges)