Add Catalan (ca) localization

[losimo]

Summary

This PR adds a complete Catalan (ca) localization for ProtonWallet, contributed as lib/l10n/locale_ca.arb.

• Translates all 617 strings from locale_en.arb.
• Follows the Softcatalà style guide (register conventions for vós/tu, quotation marks, punctuation, and terminology).
• Domain terminology was reviewed for consistency (e.g. unifying "seed phrase" / "recovery phrase" / "mnemonic" into a single Catalan term, distinguishing "passphrase" from "password", 2FA → A2F).
• All ICU plural forms and placeholders were preserved and validated against the English source (0 missing/extra keys, matching placeholder structure).

I'd be glad to see this included in the next update. I'm also happy to keep this translation up to date over time.

Note: I've also submitted a request to join Proton's official community translator program (mentioned in the project README) under albert@losimo.cat, in case that's the preferred long-term channel for this locale going forward — happy to coordinate through either path.

Test plan

• Validated locale_ca.arb is well-formed JSON
• Confirmed key set matches locale_en.arb exactly (617/617, no missing or extra keys)
• Confirmed ICU plural/placeholder structure matches the English source for every string