Fix material i18n fallback and published-project polish

[pablojimeno]

Summary

Pilot-feedback bug fixes spanning two surfaces.

Materials — When a facilitator edited a material in English, viewers in
es/it/el saw the prose sections disappear entirely (heading and all),
because the form persisted blank strings into the other locale slots and
materials_helper#material_prose used || which doesn't fall through
truthy empty strings. Fixes:

• material_prose and material_meta_description use .presence so empty
  / whitespace-only locale slots fall through to the English value.
• The Translatable concern now drops blank locale slots before validation
  for every translation column on every model that uses the concern, so the
  JSONB column never accumulates { "es" => "" } noise from forms again.

Published project page (/published/:slug) — three cosmetic regressions:

• The challenge was a tiny C6-style pill. Now renders the shared
  challenges/_card.html.erb partial, which gained a mode: local. In the
  new :public mode the question links to the challenges index (no Turbo
  preview drawer) and the bookmark + curator-edit affordances are hidden.
• Inline images uploaded via Trix could overflow the column and lacked
  caption styling. Added scoped CSS for .trix-content figure.attachment
  that clamps the image to the container width and restores figure margins.
• Removed the redundant prose prose-stone max-w-none wrapper around
  process_summary. ActionText already wraps its output in
  <div class="trix-content">, and the existing .trix-content > * + *
  rule provides paragraph spacing without conflicting with prose.

Spec lives at .munkit/specs/2026-05-08-post-pilot-fixes/.

Test plan

• bin/rails test (734 runs, 0 failures)
• bundle exec rubocop (clean)
• Manually verify on staging: open a material in Spanish that was last
  edited in English, confirm sections render with English fallback content
  (heading + body)
• Manually verify on staging: open /published/<slug> for a project
  with a challenge — challenge card renders, click goes to /challenges,
  no bookmark icon, inline images sit inside the column, multi-paragraph
  summary has visual spacing between paragraphs