feat(fase-20): accept-language + bootstrap recognise 'eu'

Extends the Fase 10.8 Accept-Language parser to surface `'eu'` as a third
known primary subtag, gated on the same q ≥ 0.5 threshold the existing
`'es'` path uses. `'en'` continues to short-circuit because it's the
source language. `AppLanguage` widens to `'en' | 'es' | 'eu'`.

Tests:
  * Unit (apps/web): LANG-U-EU-01 bare 'eu', LANG-U-EU-02 'eu-ES',
    LANG-U-EU-03 'eu-FR;q=0.5,en;q=0.4' (borderline q wins), LANG-U-EU-04
    navigator.languages array — 4 new specs, all green.
  * Integration (packages/test-utils): LANG-INT-EU-01 user updates own
    language=eu, LANG-INT-EU-02 user attempts language='fr' and gets the
    Postgres 22P02 from the ENUM (cast through `as unknown as` so the
    narrow TS type doesn't block the test).

The integration negative case lands on the language_code ENUM gate from
migration 028, not RLS — it would still fail even from a service-role
client.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

apps/web/src/lib/utils/accept-language.test.ts

@@ -51,4 +51,28 @@ describe('detectLanguage (Accept-Language style strings)', () => {
 	it('AL-12: unknown language (fr) falls back to en', () => {
 		expect(detectLanguage('fr-FR')).toBe('en');
 	});
+
+	// ─── Fase 20.4.1 — Euskera (eu) support ───────────────────────────────
+	// Mirrors the existing es-track tests. Coverage matches the plan's
+	// LANG-U-EU-01..03 identifiers.
+
+	it('LANG-U-EU-01: bare "eu" maps to eu', () => {
+		expect(detectLanguage('eu')).toBe('eu');
+	});
+
+	it('LANG-U-EU-02: regional variant "eu-ES" maps to eu', () => {
+		expect(detectLanguage('eu-ES')).toBe('eu');
+	});
+
+	it('LANG-U-EU-03: "eu-FR;q=0.5,en" — eu wins by primary preference', () => {
+		// q=0.5 is the lower bound we accept (mirrors AL-10 for es). The
+		// borderline value still beats `en` because the parser ranks by
+		// q-score and stops at the first known primary.
+		expect(detectLanguage('eu-FR;q=0.5,en;q=0.4')).toBe('eu');
+	});
+
+	it('LANG-U-EU-04: array of language tags — first eu wins over later en', () => {
+		// Mirrors AL-11. Validates the navigator.languages path for eu.
+		expect(detectLanguage(['eu-ES', 'en-GB'])).toBe('eu');
+	});
 });

apps/web/src/lib/utils/accept-language.ts

@@ -1,21 +1,24 @@
 /**
  * Detect the user's preferred app language from an Accept-Language header
- * (or navigator.languages array) — Fase 10.8.
+ * (or navigator.languages array) — Fase 10.8 + Fase 20.3.2 (eu).
  *
- * Supported app languages (in sync with messages/{en,es}.json + Paraglide
- * languageTags): `en`, `es`. Anything else falls back to `en`.
+ * Supported app languages (in sync with messages/{en,es,eu}.json +
+ * Paraglide languageTags): `en`, `es`, `eu`. Anything else falls back
+ * to `en`.
  *
- * Rules (per plan §10.8.1):
+ * Rules (per plan §10.8.1, extended for Fase 20):
  *   * Inputs we accept:
- *     - bare tag: `'es'`, `'en'`, `'es-ES'`, `'en-US'`
- *     - RFC 7231 list: `'es;q=0.9,en;q=0.8'`
- *     - `navigator.languages` array: `['es-ES', 'en-GB']`
+ *     - bare tag: `'es'`, `'en'`, `'es-ES'`, `'en-US'`, `'eu'`, `'eu-ES'`
+ *     - RFC 7231 list: `'es;q=0.9,en;q=0.8'`, `'eu-FR;q=0.5,en;q=0.4'`
+ *     - `navigator.languages` array: `['es-ES', 'en-GB']`, `['eu-ES', 'en']`
  *     - null / undefined / empty → `'en'`
- *   * If the highest q-scored entry whose primary subtag is `es` has
- *     q ≥ 0.5, the result is `'es'`. Otherwise (including unknown
- *     languages, malformed input, or no clear preference) → `'en'`.
+ *   * The first entry (sorted by q-score descending) whose primary subtag
+ *     is an app-known language wins, provided q ≥ 0.5. `en` short-circuits
+ *     immediately regardless of q-score because it's the source language.
+ *   * Otherwise (including unknown languages, malformed input, or no
+ *     clear preference) → `'en'`.
  */
-export type AppLanguage = 'en' | 'es';
+export type AppLanguage = 'en' | 'es' | 'eu';
 
 interface ParsedTag {
 	tag: string;
@@ -76,11 +79,18 @@ export function detectLanguage(
 	// Sort by q-score descending.
 	parsed.sort((a, b) => b.q - a.q);
 
-	// First app-known primary wins; if it's es with q ≥ 0.5 → es.
+	// First app-known primary wins; non-`en` primaries require q ≥ 0.5 to
+	// beat the implicit `en` fallback. `en` short-circuits because it's the
+	// source language and can never be more wrong than the user's first
+	// choice if they listed it at all. Fase 20 adds `eu` on the same gate
+	// as `es`.
 	for (const p of parsed) {
 		if (p.primary === 'es') {
 			return p.q >= 0.5 ? 'es' : 'en';
 		}
+		if (p.primary === 'eu') {
+			return p.q >= 0.5 ? 'eu' : 'en';
+		}
 		if (p.primary === 'en') return 'en';
 	}