feat(gain-web): cost / model-tier economics pane (depends #162) #175

Background

Three-way brainstorm consensus (Claude + Codex + agy 2026-05-25): the dashboard surfaces tokens saved, but not dollars saved. ContextCrawler already has the math — `src/analytics/cc_economics.rs` and `src/analytics/ccusage.rs` carry per-tier pricing for Claude (Sonnet, Opus, Haiku). The dashboard just doesn't plug into it.

Problem

The Hoff's TRAIT is cost-efficiency-focused — "evaluates component pricing against alternatives before committing". Right now the cost story lives in `gain --quota --tier 20x` text-mode. Glanceable dollar value belongs on the dashboard.

Proposed scope

New endpoint

```
GET /api/economics?tier=sonnet-4.6&period=30d
```

Returns:

```json
{
"tier": "sonnet-4.6",
"period": "30d",
"input_price_per_mtok": 3.0,
"output_price_per_mtok": 15.0,
"tokens_saved": 76099018,
"dollars_saved": 228.30,
"by_command": [
{"cmd": "contextcrawler rg", "saved": 17482753, "dollars": 52.45},
...
],
"by_day": [{"date": "2026-05-24", "dollars": 12.18}, ...]
}
```

Backed by `cc_economics::price_for_tier()` + `Tracker::get_summary_filtered` / `get_all_days`.

New dashboard pane: Cost

• Hero cards: lifetime dollars saved, last 30d dollars, dollars / day average
• Tier picker dropdown (sonnet-4.6 / opus-4.7 / haiku-4.5 / custom)
• Per-command leaderboard ranked by dollars saved (not just tokens)
• By-day spend chart (savings as positive bars in neon green; could compare against an actual spend baseline if `ccusage` data present)

Frontend

Single new `cost: async()` loader in `assets/index.html`. Reuse existing card + table CSS, no new components.

Out of scope (v1)

• Local-model pricing (Qwen / Gemma / Granite on BC-250 cluster) — own issue ([#TBD]). Needs amortised hardware cost + power model.
• Cross-tier comparison view ("this command would cost X on Opus, Y on Sonnet") — could be a v2 add to the same pane.
• Historical price changes (Anthropic price-cuts) — v2.

Effort

S - M. The math exists. Mostly endpoint + frontend pane.

• Endpoint + tests: 1 day
• Frontend pane: 1 day
• Doc + screenshot: half day

Why now

Three-of-three brainstorm models picked this as a top-tier add. The dashboard surfaces ContextCrawler's value already — the dollar overlay is what makes that value executive-shareable, not just developer-glanceable.

Dependencies

#162 (the dashboard surface) — merged or about-to-be.