Component polish: search-picker on-focus, nav_tabs pulse, multilang auto switch-handler

[mdon]

Small component-polish / DX commits.

The first two came out of the projects calendar work (BeamLabEU/phoenix_kit_projects#30 uses both, but degrades gracefully without them); the third came out of the publishing group-name translation work.

search_on_focus attr on the search picker

The SearchPicker hook has always honored data-search-on-focus (open the dropdown in browse mode when the empty input is focused/clicked — offer options before any typing), but the component never exposed it: consumers had to know the raw data attribute and pass it through the global rest. It is now a documented boolean attr rendering that same attribute; the raw form keeps working for existing call sites.

nav_tabs in-flight pulse

Tab buttons that push an event now pulse (phx-click-loading → animate-pulse) while the switch is round-tripping, so a click on a slower tab is visibly acknowledged instead of appearing dead. Navigation-style tabs are unaffected.

MultilangForm auto-handles the "switch_language" event

mount_multilang/2 now attaches a :handle_event hook (mirroring its existing debounced :handle_info hook) that calls handle_switch_language/2 and halts — so consumers no longer need their own def handle_event("switch_language", …) clause. Forgetting that clause previously crashed the LiveView on the first language-tab click, a silent footgun since mount_multilang already wired the other half of the switching protocol (the handle_info timer) automatically.

• Existing consumers' manual clauses keep working (the hook halts first; the clause becomes harmless dead code).
• Opt out with mount_multilang(socket, auto_switch_language: false) when a form switches language immediately, without the debounce (catalogue's import screen does this).
• Also documents a storage footgun: merge_translatable_params/4 / put_language_data/3 take over the schema's data JSONB (restructuring it into a per-language shape), so schemas whose data already holds unrelated keys must not point the helper at data — use translatable_field's settings-translations mode instead.

Testing

• mix test test/phoenix_kit_web/components/core/search_picker_test.exs — 6 tests, 0 failures (rebased onto current main, full compile clean with --warnings-as-errors).
• Browser-verified through the projects module: browse-on-focus lists people via the documented attr, and the project List/Timeline/Calendar tab switches pulse while loading.
• MultilangForm change: compiles clean (--warnings-as-errors); language-tab switching verified in-app via the publishing group-name edit form (tabs switch without any per-consumer switch_language handler). All existing multilang consumers across the workspace delegate to handle_switch_language/2, so the auto-hook is behaviour-preserving for them; the one immediate-switch consumer (catalogue import) opts out.

[ddon]

Merged and took into work and release