Cost UI: dedicated card, burst honors editor, Load sample button

[aakri0]

Why

Three related changes around the Invoke tab:

1. Cost was crammed into the Latest result card. Worth its own card so the breakdown + 30-day total have room to breathe.
2. The burst (`Invoke ×50`) ignored the editor — it hardcoded `'import os; print(...)'`, which made the load-test feature useless for anything except testing that one snippet.
3. Empty editor on first load — there's no quick path to a load-test-ready snippet. Existing per-sample link buttons (`hello / compute / sleep / fail / payload`) are buried up near the language picker.

Changes

Cost card

• New `#cost-card` next to Latest result. Three sections:
  • Estimated max cost (worst-case for the just-submitted job; populated from `/submit`'s `estimated_cost` field, with the existing fallback to `/jobs/estimate` if the API is stale).
  • Actual cost + a four-row breakdown: compute / memory / request / network.
  • Last 30 days rolling total + jobs counted.
• New `refreshUsageTotal()` calls `/usage/cost?window_days=30` on app boot, after every submit, and after every burst — so the running total reflects load-test traffic in real time without needing to open the Account modal.

Burst honors editor

• `onBurst()` now calls `buildSubmitBody()` — same shape as the regular `Invoke`. Each of the 50 jobs runs whatever's in the editor (language, files, payload, timeout, memory, retries, auto-install).
• 50× cost is included automatically in the 30-day total because `/usage/cost` already aggregates by owner.

Load sample button

• `#btn-load-sample` next to Invoke / Burst loads `SAMPLES[].compute` into the editor and flips to inline mode.
• The `compute` sample (sum of squares 0..200000) is short, deterministic, and substantial enough to produce meaningful timing under a load test.

Cache busts

• `app.js?v=15`, `styles.css?v=12`.

Verified locally

• Estimate populated immediately on submit; actual cost + breakdown filled in once the job completes.
• 30-day total updates after a single submit (1 job, `$0.000002660`) and again after a burst.
• Burst with the Python `compute` sample loaded — 50 jobs run that exact snippet (no more hardcoded bytes).
• Load sample button switches the editor between languages cleanly.