Widen TableDefault class attr to :any so class={[...]} idiom works

[mdon]

Summary

The seven attr :class, :string declarations on the
PhoenixKitWeb.Components.Core.TableDefault.* family reject
Phoenix-idiomatic class={[...]} at compile time, but every
component already accepts lists internally — they wrap @class
inside another list that Phoenix flattens. The :string declaration
is a stale guard from before the internal usage settled.

Surfaced when phoenix_kit_ai's endpoint list table uses a
conditional-class list:

warning: attribute "class" in component
PhoenixKitWeb.Components.Core.TableDefault.table_default_row/1
must be a :string, got: [if !endpoint.enabled do "opacity-60" end, ...]
endpoints.html.heex:241

Fix applied uniformly to all seven components in the file
(table_default/1, table_default_header/1, table_default_row/1,
table_default_body/1, table_default_cell/1,
table_default_header_cell/1, table_default_search/1) so future
consumers don't relitigate the same warning per-component. Defaults
unchanged (still string literals).

Notes

• No behavior change for existing callers passing strings.
• New behavior: class={[...]} and class={["a", false, "b"]} now
  compile cleanly, with Phoenix's standard nil/false filtering and
  flattening applied at render time.

Test plan

• mix compile --warnings-as-errors clean
• mix format --check-formatted clean
• mix credo --strict clean on touched file

[mdon]

Folding into #565 — bundling phoenix_kit follow-ups into one PR per module per session. The same one-line commit will be cherry-picked there.