fix(ui): keep field errors red inside dialog sections

[ValentinKolb]

What changes

Field errors inside a PanelDialog.Section are red again. Before, every field
error in a section body rendered in the muted grey of the section subtitle,
with an extra top margin, so a validation message looked like a hint. Field
descriptions there had the same problem.

The same applies to fields placed in the AppOverview.Main toolbar and in a
DetailPanel section's actions: their descriptions and errors now keep the
standalone field styles.

Other paragraphs in these places (app hints, notices, paragraphs styled with
utility classes) also keep their own styles now instead of being forced to the
muted 11 or 12 px subtitle look. Section subtitles and panel descriptions look
exactly as before.

This answers a request from another Cloud application, where a form inside a
dialog section showed its validation errors in grey.

Why

.k2b-ui .k2b-panel-dialog__section p was meant for the section subtitle but
matched every paragraph in the section, including the body. It is more
specific than .k2b-ui .k2b-field__error, and the stylesheet is unlayered, so
it also beat utility classes and notice styles. AppOverview and
DetailPanel used the same bare p selector in their headers, which reached
into fields placed in their toolbar and actions slots.

The subtitle and the two header descriptions now have their own classes, and
only those are styled.

Documentation

The UI context pages for PanelDialog, AppOverview, and DetailPanel
(docs-site/src/ui/context/layout/) now say that only the container's own
subtitle or description is muted and that fields in the body or slots keep
their own description and error styles.

Verification

• New browser test container-field-parts.browser.test.ts: places a field
  with a description and an error inside a PanelDialog.Section, the
  AppOverview toolbar, and a DetailPanel section's actions, and compares
  their computed color, size, and margin with a standalone field. It also
  checks that a paragraph styled by a layered utility keeps its color and size.
  With the old rules the error renders grey and the paragraph at 11 or 12 px.
• No layout shift: section subtitles and header descriptions have identical
  bounding boxes and computed styles with the stylesheet from main and from
  this branch, measured in Chromium at 1440 px and 390 px (before rebasing onto
  a tabs-only change in main).
• bun run --cwd packages/ui test: 714 tests pass, packed consumer check
  passes (after rebasing onto current main).
• bun run check: 18/18 rules pass.
• git diff --check is clean.