Add drop-in theme system with retro, grimoire, and midnight themes

[EnriqueCanals]

Supersedes #152. This PR replaces the original opt-in theme proposal with a drop-in theme system: a community theme is one self-contained folder under app/themes/<name>/, activated via ABBEY_THEME=<name>, with zero edits to central application files.

Why a new PR instead of #152

#152 introduced opt-in theming, but themes still required edits across 5+ central files (themes.rb, application_helper.rb, layouts, central Tailwind config, etc.), duplicated ~50–120 lines of layout boilerplate per theme, and leaked per-theme Tailwind tokens into the default bundle.

This PR delivers the same user-facing goal — opt-in themes that leave the default site unchanged — with a structure designed for community contributions:

• Self-contained themes: app/themes/<name>/{theme.rb, assets/, views/}
• Manifest-driven chrome: shared _abbey_chrome.html.erb partial reads from Abbey::Theme.register
• Isolated Tailwind bundles: per-theme assets/tailwind.css + @source not "../../themes" in the default build
• Generator: bin/rails g abbey:theme NAME [--minimal] [--from=retro]
• Docs: docs/THEMES.md (authoring guide) + docs/THEMES_API.md (manifest API)

Built-in themes

Theme	Description
default	Unchanged behavior — same HTML, stylesheets, and markdown renderer
retro	Memphis / 8-bit / CRT aesthetic (from #152, now in app/themes/retro/)
grimoire	Retro hacker dark-fantasy variant
midnight	Minimal sample theme demonstrating a "30-second recolor"

Architecture (5 phases, 15 commits)

1. Registry + folder consolidation — Abbey::Theme registry, manifests, git mv retro/grimoire into app/themes/<name>/
2. Per-theme Tailwind — isolated bundles via themes:tailwind:build, bundle isolation regression test
3. Shared chrome partial — theme layouts collapse to 3 lines
4. Generator — rails g abbey:theme with --minimal / --from flags
5. Docs + midnight sample — authoring guide, API reference, proof-of-concept theme

Compatibility

• ABBEY_THEME=default (or unset): byte-for-byte identical default behavior
• No new runtime gems beyond what upstream already ships
• Rebased onto current main (includes chore: batch update 5 dependencies #155 dependency batch)
• System-test selectors preserved across all themes

Test plan

• bin/rails test — 46 runs, 283 assertions, 0 failures
• bundle exec rubocop — clean
• bin/rails tailwindcss:build — emits tailwind.css + per-theme bundles
• Bundle isolation regression test (theme_bundle_isolation_test.rb)
• Generator smoke test (test/generators/abbey/theme_generator_test.rb)
• Manual visual smoke test: ABBEY_THEME={retro,grimoire,midnight} bin/dev
• Reviewer pass on docs/THEMES.md

Try it

git fetch origin feature/drop-in-themes
git checkout feature/drop-in-themes
bundle install
bin/rails db:migrate
ABBEY_THEME=retro bin/dev

Notes for reviewer

• The 15 commits are grouped by phase and each phase is independently reviewable if you prefer to land incrementally.
• Phase 1 (registry + consolidation) alone is the meaningful infrastructure improvement; the rest builds on it.
• Happy to split into stacked PRs if that's easier — just say the word.

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