feat(fase-20): eu.json seed + Paraglide config + locale parity check

Seeds apps/web/messages/eu.json with hand-translated Basque values for
every key in en.json/es.json (362 keys, $schema excluded). Each value
carries the ` [needs review]` suffix marker per plan §20.2.1 so a
native speaker can sweep through and remove markers in a post-release
pass. ICU placeholders ({name}, {count}, {n}, {version}, …) preserved
verbatim — none move position relative to the source value.

Adds `eu` to project.inlang/settings.json so the Paraglide vite plugin
emits `src/lib/paraglide/messages/eu.js` on next build, and the runtime
`availableLanguageTags` widens to ["en","es","eu"]. The compile output
is gitignored as before; verified by re-running the compile CLI locally.

New `scripts/check-locales.mjs` (dependency-free Node script) compares
every non-source bundle's key set against en.json and fails on any
missing/extra keys. Currently: eu = 362 keys, es = 362 keys, both match.

Note on translation provenance: machine/LLM seed at mid-2026 quality.
The plan calls out euskera SOV ordering risks around placeholders —
spot-checked the 12 keys with placeholders ({name}, {count}, {n},
{version}, {date}, {commit}, {word}, {title}, {days}, {lists}, {tasks},
{notes}, {section}); placeholders stayed in source-comparable positions.

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

apps/web/project.inlang/settings.json

@@ -1,7 +1,7 @@
 {
 	"$schema": "https://inlang.com/schema/project-settings",
 	"sourceLanguageTag": "en",
-	"languageTags": ["en", "es"],
+	"languageTags": ["en", "es", "eu"],
 	"modules": [
 		"https://cdn.jsdelivr.net/npm/@inlang/plugin-message-format@4/dist/index.js",
 		"https://cdn.jsdelivr.net/npm/@inlang/plugin-m-function-matcher@1/dist/index.js"