feat: localization (en/es/fr/de) — supersedes #11

[KadenThomp36]

Closes #10. Supersedes PR #11.

Summary

Adds embedded localization for the card UI in English, Spanish, French, and German.

Spanish, French, and German translations were originally contributed by @b0rv3g4r4 on PR #11. PR #11 went stale (last activity 2026-03-06) and conflicts with main; this PR carries that work forward, addresses the four review comments left on the original PR, and rebases against the current code.

Changes vs. PR #11

PR #11 issue (from review)	Resolution in this PR
Used `getConfigForm()` (legacy) — conflicts with v2.6.1's switch to `getConfigElement()`	Rebased against current LitElement-based editor (`getConfigElement()`)
Async `fetch()` of JSON locale files adds first-render latency, brittle `document.currentScript` URL detection, requires HACS `dist/` restructuring	Translations embedded as a `TRANSLATIONS` const. No fetch, no async init, no HACS restructuring
Removed the `setConfig` entity validation	Validation preserved
Distribution setup needed (`dist/` directory)	Not needed — single-file distribution unchanged

What's translated

• Overall status badge (Excellent / Good / Moderate / Fair / Poor / etc.)
• Recommendation strip (title + dynamic subtitle with `{value}` interpolation)
• Radon advisory title
• Visual editor labels and section titles (Additional Sensors / Outdoor Sensors / Advanced)

Not yet translated (will land in a follow-up so this PR stays scoped):

• Per-sensor status labels (Safe / Low / Moderate / High / Dangerous) — these are produced via the `METRIC_DEFS` table from feat: per-metric custom thresholds #29, easier to translate once that's merged
• Radon advisory subtitle prose (e.g. "EPA recommends mitigation above…") — partial translation only
• Graph axis labels (CO, CO₂, etc.) — chemical names, mostly the same across languages

HA-native approach

• `hass.locale.language` first, falls back to deprecated `hass.language` for older HA versions, then to `'en'`. Per HA dev docs and the `localize`/`locale` HomeAssistant interface.
• Regional codes are stripped (e.g. `es-MX` → `es`).
• Optional `language` config option for explicit override.
• English is the fallback for any key missing in a translated locale (so contributors can add new languages incrementally).

YAML example

```yaml
type: custom:air-quality-card
co2_entity: sensor.air_quality_co2
language: es # or 'en' / 'fr' / 'de' / 'auto' (default)
```

Test plan

• `node test.js` — 235 passed, 0 failed (29 new tests covering: language resolution precedence, `hass.locale.language` vs `hass.language` fallback, regional code stripping, unknown-language fallback, per-language status text, recommendation key→text mapping, icon backward-compat for English text, `_ts` interpolation in all 4 languages)
• All previous tests pass unchanged — English fallback preserves the existing public API
• Manual smoke test: set HA frontend language to Spanish, load card, confirm status badge shows "Excelente"/"Bueno"/etc.
• Manual smoke test: `language: fr` config, confirm overrides HA language
• Manual smoke test: editor labels and section titles render in the active language

Contributor credit

Spanish/French/German translations: @b0rv3g4r4 (PR #11). I'll @-mention them when merging so they're credited on the release.

Notes for maintainer

• No `CARD_VERSION` bump — rolls into rc2
• Conflicts expected with PR feat: per-metric custom thresholds #29 (custom thresholds) in the area of status helpers — resolution: this PR translates the user-facing strings, feat: per-metric custom thresholds #29's METRIC_DEFS supplies the keys