Cumulative duration billing + cost strip at top of Invoke tab

[aakri0]

Two improvements packaged together — both touch the cost story.

1. Pricing accuracy: cumulative duration

Bug. The worker rewrote `duration_seconds` on every terminal write, so a job that retried 3× at 5s each finished with `duration_seconds=5`, not 20. Pricing's `compute` and `memory` charges multiplied that one-attempt time, silently undercharging retried jobs by a factor of `attempts`. The per-request fee was correct (`attempts × per_request`), but compute/memory drifted.

Fix. The worker now reads the prior `duration_seconds` at the top of `handle_job`, adds this attempt's wall-clock, and writes the sum on every terminal and retry path. The field is a running total. Pricing keeps reading `duration_seconds` as before — it's now correct.

Verified end-to-end: a 4-attempt failed job (`raise Exception("boom")`, `max_retries=3`) reports:

• `duration_seconds = 0.358` (sum of all four attempts)
• `compute = 4.296e-06` = 0.358s × 0.5 vCPU × $0.000024 ✓
• `request = 1.6e-06` = 4 × $0.0000004 ✓
• total `$0.000006120` (~4× what the old code would have charged)

A new pricing test (`test_actual_cost_compute_uses_cumulative_duration`) pins the contract.

2. Cost moved to a horizontal strip at the top of Invoke

Why. Cost is what the user is reasoning about while configuring a job — should it be the first thing they see, not a square card competing with the result panel for attention. The previous big card was overweight for what's three numbers.

Layout. Slim full-width strip at the very top of the Invoke tab:

```
[ Estimated max ] [ Actual (· details ↓) ] [ Last 30 days · N jobs ]
[ $0.000507 ] [ $0.000003 ] [ $0.012345 · 73 ]
```

Click the Actual cell to expand a thin breakdown row underneath:

```
[ compute $0.000002 memory $0.000000 request $0.000000 network $0.000000 ]
```

Hidden until the user has data to look at. Strip stays compact when not in use.

Test plan

• `pytest -q` — 171 passed (one new pricing test).
• Live single submit: estimate appears immediately, actual + breakdown after the run.
• Live 4-attempt failed job: cumulative duration in the response, compute charge against the full sum.
• Burst 50 (with the `Load sample` compute snippet): `Last 30 days · 50 jobs` updates after the burst.
• Click Actual cell: breakdown expands/collapses; hidden again when a new submit clears state.

Bumps: `app.js?v=16`, `styles.css?v=13`.