fix(levelcode): show account activity spend in credits, not dollars

[ndemianc]

The activity panel on the account page still showed per-day and per-model spend in dollars, directly below a balance now denominated in credits — two units on one page for the same quantity.

⚠️ Deploy order

Requires thin.ly #383 first, which makes /account/activity return RETAIL micro-$. Until that ships, these figures are computed from raw wire COST and understate what the user actually spent by the margin. Do not deploy this ahead of it.

Details

Formatting matches AccountPage's rate() exactly — whole credits from 10 up, one decimal below, and a <0.1 floor so a cheap day never reads as free. Locale is pinned to en-US for the same reason as the balance: a bare toLocaleString() renders one number three ways across machines.

The column header and the day-detail line now say credits rather than showing $.

The admin panel is deliberately NOT converted

Its figures carry no retail conversion — they are real COGS, what we pay providers. Credits are a retail unit (≈2× cost), so showing them there would conflate what we pay with what the customer is charged, and destroy the only view where margin is legible.

Verified: tsc --noEmit clean.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR updates the Levelcode account activity heatmap panel to display spend in credits (consistent with the credit-denominated balance) rather than $, and stabilizes number formatting by pinning numeric locale to en-US.

Changes:

• Convert /account/activity cost_micros values into credits for display (including small-amount floor formatting like <0.1).
• Pin toLocaleString calls in this panel to en-US to avoid per-machine numeric formatting differences.
• Update UI labels to say credits (day detail line + table header) instead of cost/$.

Comments suppressed due to low confidence (2)

src/levelcode/pages/ContributionsHeatmap.tsx:215

• If you rename fmtCost to fmtCredits (recommended), update this call site to keep the component compiling and the naming consistent with the displayed unit.

              {fmtCost(selectedDay.cost_micros)} credits

src/levelcode/pages/ContributionsHeatmap.tsx:245

• If you rename fmtCost to fmtCredits (recommended), remember to update the per-model table body as well; otherwise this column will still call fmtCost and the helper name won’t match the “credits” header.

                  <th className="py-1.5 px-3 text-right font-normal">credits</th>

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