Serialize datetime-like values in _orjson_default - #461

## What does this PR do?

Lifts the kaleido pin in `docs/` and `examples/` from the deprecated
0.2.x line to `>=1.4`, and refreshes `uv.lock`.

Closes #938

The issue's acceptance criteria offered two routes: fix the plotter, or
"confirm kaleido >1.3.x fixed the regression upstream". The second one
is now met.

@egordm's analysis pinned the bug to `_orjson_default` in
`kaleido/_kaleido_tab/_tab.py`, which only handled `.tolist()` and
raised on `pd.Timestamp`, and noted that no upstream PR existed yet. One
does now: plotly/Kaleido#461 by @gaoflow adds an `isoformat()` fallback
for datetime-like objects, shipped in kaleido v1.4.0 on 2026-08-31. The
fix itself is theirs; what this PR adds is the verification and the
lift.

Because it sits in the central serializer rather than at a call site, it
covers the paths the plotter-side workaround could not reach, which is
why #942 reverted `_normalize_x_data` and constrained the dependency
instead.

### Verification

**1. The call sites, 1.3.0 versus 1.4.0.** Same script, same plotly
7.1.0 and pandas 3.0.5, only kaleido differs:

| Figure content | kaleido 1.3.0 | kaleido 1.4.0 |
| --- | --- | --- |
| quantile-fill band (the repro in #938) | `TypeError: Type is not JSON
serializable: Timestamp` | exports |
| `add_vrect` with Timestamp bounds | same `TypeError` | exports |
| `layout.xaxis.range` with Timestamps | same `TypeError` | exports |
| annotation anchored at a Timestamp | same `TypeError` | exports |

The last three are the call sites #938 lists as not covered by the
workaround.

**2. The real plotter.** `ForecastTimeSeriesPlotter` with measurements,
a forecast and P10/P50/P90 bands, exported through `pio.to_image`,
succeeds on 1.4.0 (60054 bytes). It also succeeds on 1.3.0, because
`_add_single_quantile_polygon` already keeps `x` as a `DatetimeIndex`.
That workaround is left untouched: it costs nothing and keeps the export
robust against future regressions, as the issue describes.

**3. The docs build.** `docs-check` run on my fork against this branch:
`uv sync --frozen` then `poe docs`, green on ubuntu-latest, 24.7 MB of
HTML uploaded. `nb_execution_mode` is `cache` and a fresh runner has no
cache, so the tutorials were executed and their figures exported through
kaleido 1.4.0 rather than replayed.

### Why `>=1.4` and not `>=1.0`

1.0 through 1.3 still carry the Timestamp bug, so the floor has to be
the release containing plotly/Kaleido#461.

This also resolves the other two reasons the pin mentioned: v1 is the
maintained line, so the deprecated v0 goes away, and 1.4.0 ships a
single `py3-none-any` wheel, so the missing macOS arm64 wheel that ruled
out `0.2.1.post1` is no longer a factor. Note that v1 does not bundle a
browser and uses a system Chrome or Chromium, which the CI runner
already has, as the green docs build shows.

## Type of change

- [ ] Bug fix
- [ ] New feature
- [ ] Breaking change (see checklist below)
- [ ] Documentation
- [x] Refactor / chore / CI

## Breaking changes checklist

- [ ] Public API, config schema, or serialized/pickled objects changed
in a way that affects existing users

Not applicable: this only moves a docs/tutorial build dependency. No
library code changes.

## AI disclosure

- [ ] No AI assistance was used (beyond grammar/spelling)
- [x] AI assistance was used — tool(s): Claude Opus 5, as a research and
drafting aid under my direction
- [x] I have reviewed, understand, and can explain all AI-generated code
in this PR
- [x] This is disclosed in a commit message (`Assisted-by: Claude Opus
5`)

## Checklist

- [ ] `poe all --check` passes locally
- [ ] Tests added/updated for the change
- [ ] Documentation updated (docstrings, user guide, examples) if needed
- [x] Commits are signed off per our DCO (`git commit -s`)
- [x] PR title follows Conventional Commits

On the two unchecked boxes: there is no code change to test, and the
upstream behaviour is already covered by `test_orjson_encoder.py` in
plotly/Kaleido#461. I ran the docs build rather than `poe all --check`,
since that is the job this pin affects; happy to run the full check if
you would like it on the record.

---------

Signed-off-by: Jakub Krasuski <kubamax4@gmail.com>