Add :per_translation_urls attr to core LanguageSwitcher

[mdon]

Summary

Lets a feature module supply explicit per-translation URLs that
override the locale-rewrite default in
PhoenixKitWeb.Components.Core.LanguageSwitcher. Pairs with
BeamLabEU/phoenix_kit_publishing#15.

Why

The core switcher's locale-rewrite default works when languages
share a URL structure that differs only by the :locale segment.
It breaks when a feature module computes per-translation canonical
URLs that aren't simple substitutions — most notably publishing's
per-language URL slugs, where /en/blog/my-post and
/fr/blog/mon-article aren't related by segment swap.

Publishing already builds the right per-translation URLs for its
in-page switcher and now exposes them on the conn under
:phoenix_kit_publishing_translations (in the matching publishing
PR). The host's root layout passes that list to the core switcher
via the new :per_translation_urls attr, and each language renders
with its publishing URL instead of the locale-rewrite default.

<.language_switcher_dropdowncurrent_locale={@current_locale}current_path={@url_path}per_translation_urls={assigns[:phoenix_kit_publishing_translations]}/>

When the assign is nil (non-publishing pages), the switcher falls
back to its existing default — fully backward compatible.

How

A new private resolve_url/3 helper replaces the eight
generate_base_code_url/2 call sites in the dropdown / buttons /
inline templates. It looks up the language's base_code against the
supplied per_translation_urls list (atom or string keyed —
publishing's atom-keyed shape is the typical caller), normalizing
full dialect codes ("en-US") to their base ("en") via
DialectMapper.extract_base/1. On a miss it falls through to
generate_base_code_url/2 per-language, so URLs without a
publishing entry continue to use the default.

Backward compatibility

The new attr defaults to nil, so any existing caller that doesn't
pass it gets identical pre-feature behavior. No host integration is
required to keep current installs working.

Verification

mix format --check-formatted # clean
mix credo --strict # clean
mix test # 1112 tests, 5 failures
(all pre-existing — V107 migration tests +
a permissions test, unrelated to this change)

Verified pre-existing nature by stash-and-rerun against the diff.
The 7 new tests in
test/phoenix_kit_web/components/core/language_switcher_test.exs
all pass; previously there was no test file for
Components.Core.LanguageSwitcher.

Test plan

• mix precommit clean
• 7 new tests cover atom-keyed and string-keyed input lists, the
  full-dialect-to-base-code normalization, fallback per-language
  when an entry isn't in the list, nil/empty/missing-attr
  pass-through to the historical default
• No behavior change when per_translation_urls isn't passed

Coordination

Doesn't strictly require the matching publishing PR to ship —
hosts that don't use publishing get backward-compatible behavior
(attr defaults to nil, no override). The publishing PR is what
populates the conn assign that hosts can then thread through.

Files

• lib/phoenix_kit_web/components/core/language_switcher.ex (3 new attrs across the dropdown/buttons/inline variants, 1 new private resolve_url/3 helper, 8 call-site replacements, 2 helpers entry_base_code/1 and entry_url/1)
• test/phoenix_kit_web/components/core/language_switcher_test.exs (NEW, 7 tests)