Split the common translations into files by topic #644

Summary

Split the common translation file into files by topic, each owning its own texts. Where each file lives, shared or in a module, is decided afterwards in a separate ticket.

Why

common holds 72 keys from unrelated areas: pages, auth, account, posts, comments and generic buttons. Two thirds are used only by the Web module. A reader cannot tell who owns a key, and a module cannot carry its own texts, because there is nothing to move except the whole file.

Scope

• Split common into topic files, starting from this draft: pages, auth, account, posts, comments, actions, pagination, lang.
• Update every _t('common.…') and t('common.…') call in the DemoWeb and DemoApi templates to the new keys.
• Keep the Spanish and English files in step.

Non-breaking constraint (3.1.x)

The framework core reads some keys by name: common.activate_account, common.reset_password, common.otp, common.pagination.next, common.pagination.prev, and common.<field> for the validator labels. Those keys keep their names in common in this ticket. Moving them is for 3.2.0.

Out of scope

• Where each file lives (shared or in a module).
• New texts. The star.* texts of Add a GitHub star message and repository link to the DemoWeb About page #542 already follow this layout.

Acceptance criteria

• every key lives in exactly one file, in both languages
• every key used by a template or by the core exists
• a test checks that every translation key used in a template exists in its lang files
• no behavior change; composer test, composer cs:check, composer phpstan and composer rector:check pass