feat(i18n): add Catalan (ca) locale

[jaumemir]

Summary

Adds Catalan (ca) as a supported locale, following the i18next migration introduced in #331.

• lib/i18n/locales/ca.json — 787 translation keys covering all namespaces: common, toolbar, export, chat, stage, whiteboard, quiz, roundtable, pbl, generation, settings, profile, media
• lib/i18n/locales.ts — registers { code: 'ca', label: 'Català', shortLabel: 'CA' }
• lib/audio/constants.ts — adds Azure TTS voices ca-ES-AlbaNeural (female) and ca-ES-EnricNeural (male)

Translation notes

Key	Translation choice
profile.defaultNickname	"Estudiant" — warm, gender-neutral Catalan term for a learner
home.greetingWithName	"Hola, {{name}}" — natural Catalan greeting that keeps the name visible as a call-to-action
profile.bioPlaceholder	Explains to the user that the bio helps the AI teacher personalise lessons

Test plan

• Switch UI language to CA via the header language selector — all interface strings appear in Catalan
• Azure TTS provider: ca-ES-AlbaNeural and ca-ES-EnricNeural appear in the voice list
• pnpm build passes with no TypeScript errors
• pnpm lint passes

🤖 Generated with Claude Code

[wyuc]

Nice work, clean PR.

Two things before merging:

1. Rebase onto main — 4 PRs landed since this branch (#409, #411, #412, #418), adding 28 new keys (accessCode.*, import.*, export.classroomZip*, and several settings.* keys). Also, toolbar.languageHint and pbl.chat.welcomeMessage were removed in #412, so drop those after rebasing.

2. Locale code (optional) — existing locales use region codes (en-US, ja-JP, etc.). Consider ca-ES + ca-ES.json for consistency. Not a blocker.