Fix cosmetic issues from PR #342 reviews

[timujinne]

Summary

• Remove redundant <div class="mb-4"> wrapper around back buttons in 4 admin pages to match the ~40 other pages
• Fix unless/else to if/else in permissions check for Credo compliance
• Fix missed responsive text classes in storage, media selector and publishing pages

Changes

Back button consistency (4 files)

• modules.html.heex, pages/pages.html.heex, maintenance/web/settings.html.heex, comments/web/settings.html.heex — back button is now a direct child of <header>, matching all other admin pages

Credo fix

• permissions.ex — replace unless/else with if/else per Credo strict rules

Responsive text classes

• Storage, media selector, publishing templates — add missing responsive sm:text-* classes

Test Plan

• mix compile --warnings-as-errors passes
• mix format clean
• mix credo --strict passes
• mix dialyzer passes
• Visual check: back buttons render correctly without extra margin wrapper