Add PhoenixKit.Modules.AI.Translation + language-switcher ai_translate attr

[mdon]

Summary

First of three PRs for AI-driven translation across PhoenixKit modules. This core PR adds the shared orchestration layer + the language-switcher UI affordance. Sibling PRs follow:

• phoenix_kit_projects — TranslateProjectWorker using this helper, wires the switcher's ai_translate attr into project/template/task forms.
• phoenix_kit_publishing (cleanup, separate timeline) — collapse the 939-line TranslatePostWorker by delegating its AI-call + structured-parse halves to this helper.

What's new

PhoenixKit.Modules.AI (lib/modules/ai/ai.ex)

Utility namespace. available?/0 returns true when the optional PhoenixKitAI plugin is loaded and exports ask_with_prompt/4. Host call sites gate AI-driven UI on this so apps without the plugin get the regular non-AI behavior automatically. Explicitly documented as a module-loadability check — not a runtime-readiness check.

PhoenixKit.Modules.AI.Translation (lib/modules/ai/translation.ex)

The shared orchestration:

• translate_fields/6 — %{field_name => text} in, same shape out, or {:error, atom_or_tuple} covering every failure mode.
• parse_response/2 — generic ---FIELD_NAME--- parser. Public for testing; arbitrary field names, case-insensitive marker matching.
• Activity log: writes one core.ai_translation.requested row per dispatched request — unified audit trail of AI token spend across consumers. Each consumer still logs its own per-resource action.
• Compile-time @compile {:no_warn_undefined, [{PhoenixKitAI, :ask_with_prompt, 4}]} — narrow MFA target so typos in any other PhoenixKitAI call still warn.

ai_translate attr on language_switcher_dropdown

New optional map attr (default: nil → today's behavior unchanged):

```elixir
%{
enabled: true,
event: "translate_lang", # phx-click target on the host LV
missing: ["es", "de"], # base codes lacking a translation
in_flight: ["es"] # show spinner, disable click
}
```

Rendering:

• Per-missing-language sparkle button next to each <a> link for languages in :missing. Click fires the host's event; the host enqueues its own translation worker.
• In-flight indicator — sparkle swapped for loading-spinner, click disabled.
• Bulk CTA — "Translate all missing (N)" footer button when ≥2 actionable languages exist (missing minus in_flight). Same event with phx-value-lang=\"*\" sentinel; host handlers branch on the value.

Component is pure event-emit. Zero PhoenixKitAI references. Buttons + inline variants don't get the AI affordance in v1.

Hardening from review (two Codex passes)

The initial commit went through two rounds of external review that surfaced 8 real issues, all fixed before this PR was opened:

Translation helper:

• Whitespace-only UUIDs (\" \") now correctly trigger :no_endpoint / :missing_prompt.
• Validation order rearranged: input validation runs before plugin-availability so callers can unit-test the input contract without a configured plugin.
• Duplicate-marker rejection: \"foo-bar\" and \"foo_bar\" both normalise to FOO_BAR — caught at entry with {:error, {:parse_error, {:duplicate_markers, [...]}}} rather than silently overwriting one field with another's translation.
• Partial-response rejection: missing field markers now return {:error, {:parse_error, {:missing_fields, [...]}}} instead of half-translated {:ok, _} results.
• Full plugin-call normalisation: try/rescue + catch :exit + catch :throw ensures every failure mode of PhoenixKitAI.ask_with_prompt/4 comes out as {:error, {:ai_error, _}} per the documented contract.

Switcher component:

• Event-name gating: enabled: true with missing/empty/whitespace event hides the affordance entirely (no dead clickable UI).
• Bulk count: subtracts in_flight from missing so a single bulk click doesn't redundantly re-enqueue running jobs.

Docs:

• available?/0 doc explicitly notes it's a module-loadability check, not a runtime-readiness check; hosts needing stricter checks query PhoenixKitAI.list_endpoints/0 directly.

Tests

• test/phoenix_kit/modules/ai/translation_test.exs — 17 tests: argument validation (5), duplicate-marker rejection, structured-response parser (9), missing-fields error (2).
• test/phoenix_kit_web/components/core/language_switcher_test.exs — 9 new tests: nil/disabled/single-missing/multiple-missing/in_flight/string-keyed/event-gating/bulk-count-subtracts-in_flight.

mix test: 1366 tests, 0 failures.
mix compile --warnings-as-errors, mix credo --strict, mix format --check-formatted, mix deps.unlock --check-unused clean.

Test plan

• CI green
• Manual: render the switcher with ai_translate: %{enabled: true, event: \"...\", missing: [\"de\"]} in any host; confirm sparkle button appears next to the missing language
• Manual: configure two missing languages; confirm the bulk CTA renders
• Manual: move one of the two to in_flight; bulk CTA disappears (only 1 actionable)